I acquired 3 amaryllis bulbs and planted them cozily in pots kept near my south-facing window (where there is also a radiator). I didn't give them fertilizer, and not one of them sent up a flower stalk. They only have the leaves. Was it because I didn't fertilize? How can I get them to flower in the future?
When you say you 'acquired' the bulbs, does that mean they were not bought brand new from a store or garden centre? If they were previously owned by somebody, they may not have been fed whilst the foliage was present on the plant - this is the only time you can fertilize bulbs, because while the foliage is present is when they're building their incipient flowers for the following year.
It may also be that being over a radiator didn't help them - no plant likes to be above a direct heat source.
Third, there is a lot of confusion between Amaryllis and Hippeastrum - what people usually call Amaryllis is actually Hippeastrum, which flowers late winter early spring in the house - Amaryllis, which has smaller flowers, produces its blooms in autumn/fall. Therefore, if what you've got there is actually a true Amaryllis, then it won't bloom now (assuming you're in the northern hemisphere).