I have a Japanese Chin who has developed a very stubborn flea problem. Have always used the topical treatments i.e., Frontline, Advantage but in the last 6 months they are not working. I changed to the pill Sentinel along with a cheaper topical over the counter drop.(Hartz) But still no improvement. She is 2 yrs (8-10 lbs) and is constantly scratching. My vet says there is no reason why these mediations are not working. Any suggestions?