re: Is your pets flea preventative no longer working?

Our dog had been on K9 Advantix for 9 years. Never had a flea problem until about a month ago. Started seeing fleas on him, gave him more Advantix, flea baths, sprayed the yard twice, vacuumed and washed everything. Fleas still. Vet and 800 Pet Meds said he may have grown immune. Not sure how that works. But switched to Nexgard. He hasn't had a problem since.