Flea treatment not working?

I discovered back in July that we have fleas. I have two cats, both short hair. I could post on here all the things I've done, but it would be several paragraphs. Suffice it to say that I have been doing everything I can possibly think of to get rid of these fleas (just today, I tossed a couch and a mattress that I think were likely infested). My question is this: I've been treating the cats with prescription topicals since I found out. Three months of Nexgard Combo (whichever one is for cats) and once with Advantage Multi. I don't think I'm imagining things, but I would swear that I'm still seeing flea eggs on my female cat. She's licked off all her fur on her stomach and the backs of her legs. I just looked at the clean sheet she's laying on today, and I'm fairly positive those white specks were flea eggs. Is this possible after four months of topical treatment?