Flea collars are a) useless and b) dangerous as they dont have quick release clips. They also shouldnt be mixed with spot ons. Dont waste your money would be my advice
When a flea jumps on a cat it takes time (upto 24hrs) for them to die, program will stop what fleas she has from laying viable eggs, it wont kill them off.
I would try a different spot on from the vet.
I must be lucky here, with 8 cats i dont get any fleas, i dont treat monthly either. Makes me wonder where all the critters go