So I know there have been tons of posts on fleas but I am trying everything. Molli seems to be allergic to fleas and is chewing herself raw

we've already been on a round of antibiotics and prednisone. Every time I think I have this flea thing under control they come back. Why won't they die?! I washed all our bedding, puppy bedding and I got a flea spray for the house. I just washed both my pups then once they were dry I put on Frontline 2 days ago. Last night I saw an alive flea crawling around on both of them. What do I do? Can I put on different flea stuff? The frontline website says not to reapply but I can't let Molli get bit and get flea allergies again. I'm so frustrated, this has been going on about 4 months but they arnt infested, I only find 1 or 2 fleas. I also live in an apartment so if they are getting them from outside theirs not much I can do. Help!!