Where Are The Bats?

We wonder where the bats are this year. We used to have several bats stay in our bat house mounted on the front of the house under the eve. Then we only had 5 then 3 and this year none. While bats are creepy little flying critters they are highly beneficial inasmuch as they scarf up the insects at night. They loved our mosquito population and we rarely ever saw one when they summered at our bat house. This year there are no bats and we wonder what happened to them. They are remarkable little creatures that can use their echo waves to locate a mosquito in the dark and take them right out of the air. They are acrobats when flying and avoid obstacles with ease.  While they are pretty creepy little critters we sure do miss having them around. Possibly a winter cave that they resided in collapsed or they died of other causes which has kept them from coming back but we actually miss their presence.