I answered a similar question a few months ago so please check that out.

But in a nutshell, it was difficult to forge convincing papers. There was sort of an arms race between slaves and those who supported slavery, so that whenever slaves could figure out ways to circumvent obstacles to escape, additional obstacles were erected. So it was illegal for slaves to learn how to read and write in most places, making it very difficult to forge papers. Also, they'd have to see the papers to know how to forge them--the language, the style, the handwriting. Doing those things takes training, so even when slaves and their allies might know how to write, that doesn't mean they could achieve all of the tasks necessary to plausibly forge free papers. Plus, paper in and of itself was a luxury, as was ink, and presented another obstacle to obtain. States started using free papers on pre-printed forms in the 19th century, so you'd need a blank form, representing another obstacle. You'd also need clothes that marked you as a free person. Slaves typically had inferior clothes that marked them as slaves.

It was a lot easier for white people to create such forms, but still a challenge--there were official seals to create, and that's extremely difficult. Yet it doesn't appear to have been done all that often. The "Underground Railroad" was somewhat of an ad hoc group of people, not a true organization with extensive planning. While northern abolitionists were there to help slaves who had escaped and made it north, they didn't go south to start helping people escape, except, most notably, Harriet Tubman.

Frederick Douglass, in his autobiography, explains a bit about how he escaped. Despite being able to read and having some money, he couldn't forge free papers. Instead, what he and some others escapees did was to borrow papers from someone else, which was risky for both of them and indicated great trust on the part of the man who lent Douglass his papers. And he had his free fiancee make him clothes suitable to his status as a free man. And he was in a big city where strangers could walk without suspicion, which helped. And he was only a short trip from the border by rail, meaning his papers didn't need to be useful very long--a matter of a few hours, and they were checked just twice during the train ride.

If you were far from a free state, forget about escape. The vast majority of escapees came from border states. A trip across unknown lands, with few resources and no aid, was treacherous. If you traveled by the day with your forged free papers, you risked being taken to prison by a suspicious person, and then compared with all of the escaped slave notices on record. You also risked someone kidnapping you and destroying your free papers--it wasn't uncommon for free blacks to be kidnapped into slavery. Escaping quickly was key, because as soon as an escaped slave notice went out, you were on borrowed time. Or you could travel at night. But a black person traveling at night was suspicious, plus there were patrols on horseback and with dogs, patrolling for escaped slaves and it would be great sport to them to catch you.

In the book Gateway to Freedom, about the Underground Railroad in New York City, the author mentions that a lot of escaped slaves were caught right after they arrived in New York City--within the day. But why? What would indicate that these people were escaped slaves, in the middle of this big city? Slaves were marked in some way, in the sense of having certain clothes, acting in a certain way, that distinguished them as escaped slaves. They looked like slaves and their very appearance often raised suspicion and while all of this was going on, slave states were busy passing federal laws that would make it very easy for them to collect their escaped slaves, and very difficult for abolitionists to help them. For example, in New York, they passed a law requiring a court hearing before a suspected escaped slave could be taken back to slavery. So then a federal law was passed making those hearings rather perfunctory, plus they managed to get a pro-slavery judge in New York to hear the petitions so an escaped slave could arrive in New York, be captured, see a judge, and be returned to slavery all within the same day. It was virtually a rubber stamp that someone being seen in that court would be taken to slavery. Free papers couldn't protect you if someone said you were their slave. Abolitionists tried to fight against this, but if they didn't know there was a slave caught, they couldn't get to the courthouse to prevent extradition/kidnapping in time.

Oh, and one last word. Solomon Northup of 12 Years a Slave was born into freedom in New York, and kidnapped into slavery. His free papers did no good whatsoever. Free papers meant nothing if you lacked the ability to enforce them. Northup needed 2 wealthy white men, one a lawyer, and financing by the state of New York, to physically travel down to Louisiana and then outfox Northup's enslaver by waking up a judge in secrecy in the middle of the night to get the papers in order to free him. And then they had to go to find Northup, bringing along local authorities to back them up, and basically ambush him so he couldn't hide Northup from them, in order to get Northup's enslaver to let him go without a fight. That's a pretty large amount of effort and legal muscle to bring to bear to rescue someone who was legally free all along and had never been enslaved or escaped slavery. And no one was ever punished for his kidnapping because the law was completely on the side of his kidnappers.

So these are a few of the obstacles that existed that made forging difficult, and ultimately usually insufficient to use for escape in a large number of cases.