Frontline Plus possible skin reaction

I've had my dogs on Frontline Plus for 3 years and it seems like my Lhasa Apso is experiencing a skin reaction to it. I've noticed it over the last several months. The first time it left a scabby area between the shoulder blades that didn't go away for about 2 months. When it did finally break away it looked reallly gross (anyone remember the Botfly thread????). Now that area of skin feels thick and different. Since then I've applied Frontline Plus two more times, most recently 2 days ago. I do not put it on the spot where she had the initial scabbing. A few days after I applied it last month I noticed an area about the size of a silver dollar where the hair is gone and her skin is dry. She can't reach it as it is also betwen her shoulder blades, to the side of where her initial reaction was. My other dog has no problem with the product.

I am, however, thinking of switching to another flea/tick preventative for the above mentioned reasons and also because I recently heard that dogs can build up a resistance to flea products and it's good to switch after a while. Has anyone heard this? I am also seeking feedback on the various treatments available. I am not considering Promeris based on threads on this blog about the serious potential side effects of the product. But I have also heard that some Advantix useres have experienced adverse reactions in their dogs. I am nervous to try something new so any input is much appreciated. It must be for fleas and ticks as we are in Florida and they are so bad year round. I also heard that down here flea treatments are only lasting about 3 weeks. Has anyone else heard this? Is it bad to treat them every 3 weeks???? 

I would love to use a natural product, but I don't dare risk it as bad as we are infested here. I can treat my lawn all day long, but the minute you step off your own turf it's a problem.