I’ve had two lower incisors worked on by a local dentist. I don’t know what cement they use for this, but the damn things keep coming off! Is it me, or is the dentist just using the wrong kind of cement to hold the caps on?
Maybe you chew funny? Seriously, try asking your dentist if you can eat something in front of him/her and see if maybe you’re using your teeth in some way that can be compensated for by extra cement or a differently placed post.
Corr
I have two permanent caps on my front teeth - had them since I was 9. I haven’t had any trouble with them falling off, though.
My front teeth were knocked out at such a young age that the dentist thought my incisors and the teeth right next to my front teeth would grow bigger. But they didn’t, so despite expensive orthodontics it still looks like I have a slight overbite. We considered getting caps put on the teeth next door, but my bite falls mostly on those teeth, so it won’t work. Your dentist should be able to tell from a cast where your bite is, so that he can adjust your teeth properly.
Me, I need new caps, but it costs almost $2000. My insurance would cover half, but that’s just not enough. Anyway, I’m pretty happy with my smile right now - I used to get teased for being a big-toothed freak but now I don’t really care. (You can see my big ole smile here).
Enough about my dental history. Good luck - you might try a new dentist. Mine is widely known for her ability to make fake teeth look fabulous.
IANAD, but I thought that crowns were supposed to last for quite a while.
I had a crown done about 3 months ago and the dentist at the time told me it should last for a good couple of years and then might need re-fixing. While they were waiting for the crown to be made they fixed a temporary plastic one on, and that had a tendancy to move, but that was because it was not fixed properly so they could take it off.
I would guess that yours have not been fixed on properly, but as I said, IANAD. Perhaps it might be an idea to go and speak to another dentist and get their opinion or see if they can fix them for you.
Rick
The crowns/caps this dentist put on fell off yet again. Near as I can figure my big “geographic” tongue kept pushing at them until they loosened! 
However, when the dentist’s office assistant called to set my next appointment (which I had today), I told her about this; she said, Never mind, those were temporary. Your permanent caps are in. So I came, and brought the old caps, which the dentist didn’t need now. He used some other kind of cement to fit the new ones in, and they seem firm; besides my tongue isn’t pushing against these caps, which feel firm. These may stay in. 
Going through this myself at the moment. In exactly 24 hrs, 30 mins I will have my permanent caps put in. Temporary ones can be a nightmare. You can’t talk normally, because they wobble and make clicking noises that only you can hear. You can’t eat anything hard for fear they’ll fall out. Your tongue can’t resist having a good poke around to explore this alien presence in your mouth, thus further loosening the things. I was tempted to grab the superglue, but thought better.
If the permanent ones are any good, they should last at least 10 years, and possibly 20, provided you don’t try the old “biting the cap of a bottle” routine too often