My first question would be whether the veterinarian finds any fluid in the anal glands and what it looks like. Liquid from the anal glands should be brown, thin and see through. If it’s anything other than that, there is infection. If the liquid is more like paste then your dog should have been given antibiotics.
If your veterinarian expressed the glands and all was normal, thin, brown and liquid, then your dog may have had his anal glands expressed so many times that he can’t do it on his own. If your dog isn’t having anal gland issues, then his glands shouldn’t be messed with. It really shouldn’t be a routine part of a grooming session for a dog’s anal glands to be expressed.
Another cause is plumpness. Some plump dogs have extra plumpness in their bottom which interferes with draining their anal glands they go to the bathroom (how it’s normally supposed to work). Another possibility is allergies – basically an itchy butt.
And finally, for anyone who has ever suffered from sciatica, you understand the irritated feeling and the desire to scratch – same thing for our dogs with itchy bottoms. It’s possible these dog need to see a chiropractor just as humans suffering from sciatica.