Frontline is great, but your in Texas and it isn't a 100%. The problem with what you are describing, scabs (miliary dermatitis), hairloss (alopecial around tail end and hind legs), are classical presentations for flea allergies.
Unfortunately, it only takes the bite of one flea to start the cycle. Let say you were 4 days late on the Frontline OR you were right on time but your cat is only 100% protected for 22 days instead of 30...and a flea got the cat. A cycle of grooming and bumps develop.
With cats like this, so that we are not pilling, washing, spraying, etc. all the things they "love"...we give an coritsone type injection to stop the allergic process and I place the cats on Advantage to be given every 25 days. I think Advantage has a better kill for my flea allergy cats and unless you have a problem with ticks, Frontline is not needed. With that said, if you have 5 more doses of Frontline and your vet won't exchange...than use them. Just apply it at day 25, you might want to treat the house once "just in case" and wait for a freeze.
Fleas come in from socks and other things than animals.
Let me know if you have further questions.
Dr.V
Dr. D.A. Vamvakias
Veterinarian