Hello.
Feel so sorry for my dog - she is 6.5 years old, I've always been shit-hot with flea treatments and aside from when I got her (riddled) she has never had a problem due to being regularly treated.
I have 2 cats, both regularly treated also (and again, never had any problems).
Always used spot-on treatments, but recently (last 6 months) it is just beyond my reach, financially for all 3. I have been buying the cheaper spot-on from Pets at Home for the cats and a flea collar for dog, with a flea shampoo and regularly grooming/checks. All has been fine.
We have a stray Tom Cat hanging around, he scents everywhere, hangs around my house (both my cats are spayed/neutered) and is very unkempt. (not sure it is him but seems a coincidence) Noticed dog scratching last week, thought nothing of it (they do scratch after all). Long story short, she has been bitten to buggery on her back, scabs the lot 
Discovered the extent yesterday during grooming her. They don't look infected.
Have changed her flea collar, given her a bath with the shampoo, washed all pet/human bedding, vacuumed the house to within an inch of it's life and re-treated the cats.
My question is, she is still uncomfortable and scratching - I have searched for fleas but not seen a single one and seen no flea dirt either??? At what point would you take her to the vets? Or is this a case of the fleas being gone but the itchiness continuing?
Just in case anybody wants to pull me up on dog ownership and costs etc - I know, however very different financial circumstances currently and hopefully only temporary.