Is there a reason for an amaryllis to put up very short spikes - blooms are beautiful, but only about 6" above the ground.

Shorter flower stems are usually a result of genetic hybridizing. That means there is not much you can do about it except to enjoy the flowers regardless of their stem height.

I have on occasion had normal spikes, than 1or 2 on the same bulb be 2" high with a great bloom