According to the Smithsonian, many bats give birth to their young in the May / June timeframe. This may have some bearing on the limited number that you have observed.

Derived from the source...

For bats, daytime is for rest in some secluded shelter, hang head down, groom, sleep, and perhaps even quarrel. When night approaches, bat activity increases; they fly about inside their cave then finally emerge to forage for food and drink water. After feeding for an hour or two they may rest again, then have a second feeding before daybreak.

In many species, the males and females roost separately except when mating. In May or June the females congregate in large colonies and give birth. Most species bear only a single young per litter, but others may have two, three, or even four. The baby bat, already large and well developed, crawls to the mother's nipples, attaches itself and feeds. In the evening when the mother forages for food, she may, for the first few days, carry the young with her. Later the baby remains behind, clinging to the wall or roof of the cave or shelter. The mother may return several times during the night to feed her young.
Source(s):
http://www.si.edu/Encyclopedia_SI/nmnh/batfacts.htm


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