Fleas can develop resistance to the monthly products. A different monthly product (different ingredients) should be tried.
Capstar, a daily med, may be given to knock out adult fleas quickly. It becomes very expensive with multiple uses. My local shelter uses it in combination with Frontline Plus, which has an insect growth inhibitor to slow/prevent larvae and eggs from developing.
Combing with a flea comb and dropping the fleas, larvae and eggs into soapy water works; its just very time consuming.
A dust bath with food grade diatomaceous earth may help reduce fleas. It may be spread around in areas where no insects, grubs, or nematodes are desired. Do wear a dust mask yourself and avoid stirring it up; the exoskeletons of the diatoms can irritate the lungs or humans and critters.