I would be VERY cautious of anything bought over the counter for fleas… they are generally NOT approved or designed for use in rabbits so the manufacturers may add ingredients that are not rabbit safe. I’ve seen nasty reactions with OTC flea/tick stuff in cats, usually from owners trying to split dog doses- wrong dose + wrong species can = problem, and this is basically what you’re trying to do for a rabbit (I’ve worked in several vet offices). To be fair, the vet products are also not technically approved for rabbits (bottom line, not enough money in it for the companies to deal with regulatory stuff) but are overseen by vets familiar with their composition and use, and several products been shown to be safe in practice. Personally, for any species, I *only* trust things I can get from the vet with regard to parasite control… there’s just too much scary stuff out there
Revolution should help you but ivermectin won’t do anything for fleas- if that’s all you’re using it for then save your money
If the Revolution isn’t working you could ask your vet about Advantage (imidacloprid), which is also rabbit safe.