Originally Posted by ShaftEd
There is one other possible cause. If you push too hard while installing a new bulb, you can actually pop the adjustment arm out of it's joint. What you end up with is the light beam aim is way too high and no amount of adjustment changes it. If you did this you are pretty much up a creek, as the entire headlight assembly is sealed and you can't easily get to the adjustment arm to pop it back in place. Here is a link to someone who was able to fix this problem, but it's not easy to do.
The main thing is that when you replace bulbs, be gentle and don't push too hard to seat it.
What he said. They pop out very easily. To bad they don't pop back in as easy.