What makes amaryllis bloom?

We have 14 amaryllis bulbs. These are divisions from about 8 last year. We started with 2 or 3 in 1997. Haven't purchased any new ones. Our problem is that usually they don't bloom. Last year 2 out of 8 bloomed. This year 1 out of 14. We can understand that newly divided ones don't bloom, but that's not always the case. In any event, how old do they have to get before blooming? We let them go dormant in the winter, and re-plant them in pots with bone meal in late Feb/early March. The leaves look healthy. The bulbs grow bigger. What's the secret?

TIA
LAS

Some of mine seem to bloom better when they're crowded- I wonder if that could be the problem?

Yes, they like to be really crowded to bloom well - pack them together and see what they do next year - REALLY pack them in the pot. Let the foliage grow over the summer, fertilize and when the foliage dies back this fall, STOP watering them - stick them in a cool and dark spot and let them rest. Keep an eye on them - they have a tendency to start budding while still in the dark, cool area. Bring them into the light and see how they do.