Hi Giggles,
The only problem I have come across with Frontline spot on, is getting my 3 cats to sit still, while it's applied. Are you sure you are separating the fur on the back on neck (base of skull), and making sure that the liquid goes on to the skin, and not just on the fur. The other answer you received is quite correct, it only takes one flea to start up feline dermatitis. If using Frontline spot-on, which works well for my 3, it needs to be regularly used every 4 weeks from beginning of April to end October. October to March inclusive, it can be used every 6 weeks, as the fleas are inactive in the colder weather. That is of course, unless you keep the house quite warm with central heating, and then it needs to be every 4 weeks all year round. Hope this helps. Good Luck.