The light trick is very good advise.

I used to have several reflector-type lights that clip on the nearest 2X4 and are used in remodeling.

String together as many as you can and set them up in the daytime.

Bats leave at dusk, so wait until it is fully dark outside, then plug 'em in.

The light will shine out through holes that you otherwise may miss.

Plug all holes while the critter are out feeding. Steel wool is a great tool for that job.