From Wikipedia
"At the Ruhr pocket, two Jagdtiger commanders failed to attack an American armored column about 1.5 km (1 mile) away in daylight for fear of attracting an air attack, even though the Jagdtigers were well camouflaged. Both vehicles broke down while hurriedly withdrawing through fear of an air attack that did not come, and one was then destroyed by the crew."
"Near Unna, one Jagdtiger climbed a hill to attack five American tanks 600 meters away and below; two withdrew and the other three opened fire. The Jagdtiger took several hits, but American projectiles could not penetrate the 250 mm (9.8 in) frontal armor. However, the inexperienced German commander lost his nerve and turned around instead of backing down, exposed the thinner side armor, which was eventually penetrated and all six crew members were lost. Carius wrote that it was useless when crews were not trained or experienced enough to have the thick frontal armor facing the enemy at all times."
What I don't understand is why wouldn't you reassign your veteran tankers to these tanks instead of these green novices. Surely you would rather lose a more disposable PzIV or Stug then a very expensive and hard to replace heavy tank.
Germany had a pretty severe manpower crisis during the latter half of World War II, in addition to a pretty severe drought of mechanized vehicles. Short answer is, what experienced tankers were still around were busy at the front. New recruits would be rushed through basic training and tank school, and thrown into the fight. They missed out on a lot of the tips and tricks you pick up from veterans by serving with them, or being trained extensively by them. The Heer didn't have the luxury of regularly granting their tank crews extensive leave or R&R cycles. Oftentimes they'd stay at the front until killed, captured, or wounded badly enough to be evacuated.
The Luftwaffe was really bad about this too. German pilots flew until they were killed or captured, usually. That's why you read sources about Luftwaffe aces with hundreds and hundreds of kills, and then see that the Allies' top fighter aces kill counts numbered in the tens of planes, not hundreds. Once they started amassing kills and flew a good number of missions, Allied fighter pilots would routinely be cycled back to their home countries to train new pilots and do publicity tours, and pass on the lessons they learned.
Just to give you an idea of the disparity between the two policies, look at this wikipedia list of top ww2 fighter aces. See how far down the list you have to go until you get to the top Allied ace, Dick Bong?