Problem with fly infestation inside home randomly in the Summer ONLY

Intermittently we have been having problems with flies entering our home especially a little while after starting dinner. It appears that the smell is drawing them in from whatever their source is. Today when I arrived home I had our new shutters closed in our office. When I opened each of them there was a very large number of flies in each stuck behind a shutter and somewhat stunned. They were a little bit more lethargic then usual which I suspect is because they were trapped somewhat behind the shutters in the hot void between them and the window for most of the hot sun evening hours. I sealed the right side shutter with a black trash bag and painters tape completely encasing the shutter so that nothing inside the house can get into the window void. This is to determine if the Flies are entering three possible Gap in the screen outside and then migrating up through the channel where the windows slide up and down. If this is the case it will simply be a matter of stuffing that channel with some cheesecloth or other easily removable fabric. I put about one cup of apple cider vinegar inside of the sealed area on the right side shutter to see if that makes any difference. My understanding is that flies love apple cider vinegar so that might help to attract them in that void to see if they are entering through the window from outside.