Is flea treatment working? I treated my cat about a week ago. I used a flea comb everyday and catch 0 fleas but I still see small flea dirt.?

I treated my cat on the 15th , I have a roommate with two cats who didn’t treat hers until 2-3 days later. I’ve been spraying/cleaning/vacuuming like crazy. I used a flea comb twice a day to check my cat and have never found a flea, but I’ll still find about very small amounts of flea dirt on my white sheets (put... show more I treated my cat on the 15th , I have a roommate with two cats who didn’t treat hers until 2-3 days later. I’ve been spraying/cleaning/vacuuming like crazy. I used a flea comb twice a day to check my cat and have never found a flea, but I’ll still find about very small amounts of flea dirt on my white sheets (put down light sheets to find evidence). Is my cats medicine not working or is this flea dirt from remaining fleas that have just bit her and take some time to die? I also haven’t been seeing any dead fleas.
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