Do in door out door cats bring in fleas even while using frontline?

According to the manufacturer, Frontline is not a repellent. You may still see fleas or ticks on your cat or dog. Frontline claims that it will kill ticks and fleas and prevent them from laying eggs. Oftentimes any fleas or ticks you see may be on their last leg. I pick them off and put them in a small container of water with a small amount of bleach or dish washing liquid just to make sure they die. I also comb my pets daily to remove these disgusting bugs.

Possibly

Yes. Frontline doesn't work at all anymore in most areas, so I suggest you ask your vet what DOES work in your area.