Fleas develop an immunity to the pesticide in spot-on flea treatment over time. Switching to a similar product is dangerous and will likely not work. Considering that we now think spot-on treatments might cause seizurers and even fatalities.
You can get rid of fleas naturally, it just takes a bit more work. First use a flea comb on your dog. Then bathe her with a shampoo with neem oil which repels fleas and ticks. You can sprinke her with baking soda (the fleas basically drown in it) or a natural flea powder with DE which actually cuts the fleas in half. This can also be used around the home and yard. Wash all of your pet's bedding and all of your bedding. Vacuum the house thoroughly. Repeat this for a few days. There's also a natural squeeze-on oil from Sentry and rechargable flea collars. Many of these items can be found at www.onlynaturalpet.com. Good luck!