I have a fly problem, can't figure out why

[Steve Balinski]

Anyone ever deal with a fly problem?
I have a duplex, about 2+ weeks ago both of our units started getting flies. My unit has about 10-15 a day, the other unit has about 30+
I had Orkin come over, they didnt see anything specific that indicated where the problem was coming from. Both attics are pretty "clean", except there is evidence of a squirrel (crap) in my tenants attic, BUT no smell of anything dead, AND no swarms of flies in the attics. At this point I dont know what to do. I've been told they run their course, but going on 3 weeks seems kind of long. I'm not sure what else to do at this point.  My tenant has them all over the house, mine seems to be localized to the basement.  I put a stick trap on top of the floor drain, 0 flies on it in 3 days now, they're just all on the windows.