There is always a base failure rate with any technology. Bombs are no different in this respect (and many of them are more complicated than you might realize, and complication often adds new failure modes). So let's imagine you have a bomb that works 99.9% of the time — that is way higher than the success rate of many bombs (some modern bombs apparently have failure rates at least as high as 7% — I've no idea how high it goes today, or what it was back then) but it illustrates our purpose. How many bombs are we talking about? There was apparently around 7 million tons of bombs dropped on Vietnam by the US during the war. So if .1% of those failed, then we're talking about 70,000 tons of unexploded munitions randomly dispersed around the country. If we use more realistic failure rates, we might come up with several orders of magnitude more (with 7%, we have ~500,000 tons — note that the individual bombs usually weighed well under a ton, and with cluster munitions, you can have individual bombs that weigh practically nothing, so that weight would mean a lot of unexploded ordnance).

So the real issue is not how difficult it is to make a bomb explode — maybe you can do that really, really well. The question is, what's your total sample size? And in the case of Vietnam, it is HUGE — we dropped way more tonnage on Vietnam than was expended by all sides during World War II (which was about ~2 million tons of TNT equivalent, including the two atomic bombs). So even a very small failure rate would have left a lot of unexploded ordinance, and under jungle conditions the failure rate was probably relatively high anyway.

TLDR;: The issue is not how faulty the bombs were so much as it is how many bombs were dropped.

The thing about military ordnance of any kind is that it is never entirely fool proof, even today with advanced munitions. Bombs of all types have different criteria on what will cause them to detonate. Some are designed to detonate upon impact, some can be time delayed to detonate after impact, others are designed to detonate at a certain altitude (this is common with nuclear weapons and most modern munitions so that the ground does not absorb the detonation).

In relation to Vietnam/Laos you have to remember that these are jungle environments that are heavy in foliage, humid, with ground that is not extremely hard in many places. All of these could lead to any number of reasons as to why munitions did not detonate in that time. The ground where they landed could have been too soft and the bomb simply was absorbed by a the ground (this is also prevalent to a malfunction of the internal detonation mechanism in many cases), the temperature/climate could have caused the internal working mechanisms to malfunction thus preventing detonation, the parts within the weapon might have been faulty, the weapon might not have been stored properly, etc.

There is no simple "they didn't explode because A, B, C" answer really. Every bomb is different, every situation in which it is used is different. It just really depends on the weapon used and where it was used.

One thing that should also be kept in mind is that an awful lot of these bombs were quite old, dating back to World War II or even earlier and not necessarily stored properly in the meanwhile. This could be a contributory factor to their not detonating properly. That being said, the only issues I am aware of for sure with the older bombs is them detonating when we didn't want them to (aerodynamic issues leading to explosions soon after release from A-6 Intruders and higher heat sensitivity leading to high order cook offs on the Forrestal).