Bernice, if you haven't done it yet, I'd walk around in the outdoor area where the dog goes and see if you still have adult fleas there. If you wear white socks, you should see some fleas on them if there are any. Nematodes attack the flea larvae, but they don't do anything to the adults that were there when you treated the yard. If the dog is being reinfested with adults, either from outside or from hatches indoors, then you still need to address them. If the dog has adults on it, then it probably is dropping flea eggs indoors, so continue to clean, vacuum, and wash the bedding, dog, etc. I was thinking that you don't have carpet, but if you do, you should consider having it steam cleaned. I assume you don't take the dog out to areas beyond the house where it can collect fleas. Do other dogs/cats have access to your yard? If you can get the hatchings under control, the flea population should subside.
If you get rid of the fleas and the dog still itches, then the entire area of skin allergies is in play. Non-specific skin allergies can be hard to assess and treat, even for vets, but you probably start with considering diet, dietary supplements, topical itch treatments, and the type of shampoo that you use. There's a lot of information about pet allergies on the Web, so you could do some research there.
The fleas probably were around before the summer, so if the itching happens only in the summer and every summer, there probably are other allergies at work in addition to the fleas. The fleas make it worse, though. If the dog has a non-flea skin allergy that you simply can't identify, it could be something like a hayfever allergy to pollen or something similar. If the dog is a purebred or mostly purebred, you might consult the rescue group(s) for that breed. They've see just about everything imaginable, and they invariablly have a Web presence. If you get to the point where you have to use pharmaceuticals, you could try benadryl, but you'll have to work out the dose. I would resist the vets' tendency to give prednisone pills or shots unless the situation is quite serious and nothing else works.
Because you've had a flea infestation, you should monitor the dog for tapeworms. It is not unusual for tapeworms to accompany flea infestations. Good luck.