Could be the tank undergoes large temperature changes as well? Such a small volume of water and your heater may be broken?
Maybe, though I feel no heat coming from the heater. Besides the tank sits right in front of a room heater next to my other tanks. The temps in those tanks hover around 74-76 degrees when the room heater is on and 72 when it's off. It might be a bit hotter since the lamp is a bit close to the cover, but I don't think that would cause the shrimp to not breed. From what I've read, breeding occurs after a certain age, maybe a few years after hatching. This is probably because they are so long lived (20 years or so)