Please stop overdosing your kitten with the flea medication, it is good for one month, and even tho you see a flea on your kitten if it has been treated within the 30 days the flea or fleas will die within 24 hours of contact.
There is some place in your house or yard that the younger kitten is getting access to that the older cats might not be interested in, and this is where the fleas are breeding. It could be in a carpet, bedding, insulation, the yard etc. Keep a watch on where your kitten goes, and if he is allowed outside, that may be the problem...you might have to treat your yard.

bionic fleas....lol....maybe! can they bend girders?

just had to laugh about that....but i know it's not funny when they're attacking your buddy!!

i don't know for sure, but since people can turn up resistant or allergic to different chemical treatments, i assume dogs and cats could too.....i would have to agree about the overdosing.....be especially careful about that with a kitten.....it's a huge concern....at first opportunity i would look for another treatment....i use advantage for my fellas, and even our most recent stray who was found absolutely CAKED with fleas and their "dirt" was cured within days.....

and i never put too much stock into flea combs, but i bought one for the stray who seemed to be crazy with fleas, and i have to admit, it made a startling difference in his recovery.....

Ditto what mazer said.....you're overdosing the kitten and he's picking up the fleas somewhere. Fleas don't die immediately from Frontline so finding them on your kitten isn't a surprise. Try to find where the fleas are coming from..........

Seeing a flea is not the same thing as having an infestation.

You will see an occasional flea, if you see lots of fleas then you have problem.

And where did your get frontline? There are bootleg versions and watered down versions so you may have an ineffective dose, if you bought it from an overseas vendor online, or at the flea market (pun intended)... you may need to get some from the VET...