I'm also having a similar problem, my girl molly has all of a sudden become a flea magnet! I've had her for 12months and have had no problems with flea's until now. I live with my mother who has two dogs of her own that I haven't caught fleas on yet, i've also checked my cats and could not see any flea's either, but my girls tummy is covered with them? We've usually used Sentinel, but have recently changed as we've also caught ticks on her so we have changed to frontline (a spray, not the tubes) & i have to completely drench her in the stuff and it's still not working! I've gotten rid of all her bedding as well!