There are two diseases that may be responsible for problems with an amaryllis' stalk: red blotch or mosaic virus. Red blotch is caused by a fungus and causes red spots on all parts of the plants. The stalk will eventually bend over or even break at the point of infection. Mosaic disease returns every season, causing yellow spots on the foliage and an overall reduction in plant size. Each season the stalks will be shorter.
Management of both diseases involves digging up the bulbs. Don't compost them -- they must be destroyed.
Amaryllis stalks can be top-heavy. To avoid having the stalk bend and break from the weight of the flowers, tie the stalk loosely to a stake.