This is going to sound insulting, but can't be helped. Maybe there is something in the room that is the cause of it, like rotting food - and so creating maggots from which you get the flies.
I agree with the others & if you have fireplaces it may be a good idea to check that a bird hasn't fallen into one of them behind fires/boards & become maggoty.
Flies usually head for the highest/lightest place in the house so if you are out a lot they could be coming from downstairs without you noticing.
I keep having this problem and mine is due to birds falling down the chimney and dying. It's horrible. I've just had a quote to have covers put on the chimney pots to try and stop it happening.