The bulbs are not installed correctly. It may *seem* they are, but, they're not. I had this same problem and it kicked my *** until someone here gave me a very simple solution - have the lights turned on and pointed at a blank wall when you install the bulbs.

I know you know there is a tab and the bulb has to be seated the right way. It would seem to be installed correctly at that point, but it isn't. There is a tiny bit of wiggle room involved - you will barely touch it and it will make a big difference. So, get it seated the right way, then move it around a little until it hits the right spot. Hold it in place well until you get the spring back on it.