to the forum. If the problem is with your "yard" (I assume you mean your back garden, if you are American), and you have tried all these chemicals to no effect, then the cause of the itching is something different than fleas.
Is it hot in your area? In that case, you could have a problem with harvest mites. I believe that they are called chiggers in the USA. They are microscopic in size, so without a magnifying glass you might even miss them (so might your vet). Note that spot-on treatments do not work. Some people use Frontline spray (not spot-on), but in general this does not work either. One thing you could try is 5% ascetic vinegar. This should be applied to troubled areas, but do not let your dog lick it off.
If you can get neem oil (search on Amazon), this is a quite good mite repellent. It also discourages fleas and ticks from settling on your dogs.
If all else fails, either search your dogs skin thoroughly with a very strong magnifying glass, or ask a vet to do it. You may find the cause is smaller than you think. Some biting bugs are not visible to the naked eye.
Every best wish to you!