Grooming issues can arise for several reasons.
The dogs and cats may have had a bad experience at a grooming parlour, may have been roughly handled as a pup, may be body sensitive due to injury or disease or may simply be unaccustomed to being touched over every part of his body.
As behaviours are linked, dogs that have grooming issues may also find containment difficult (which includes being on a lead or wearing a harness or dog coat), and usually dislike being towel dried.
They may also be timid or defensive around strangers and other dogs and fearful of being examined by a vet.
Keep each session short and do not try to achieve too much in one go.
Several short sessions are going to be far more effective than one long session.
Ensure that you watch your pets responses at all times and if your pet is concerned about being touched on a certain part of his body, go back to the places where hand contact was more acceptable.
Build your pets confidence over a few short sessions and try to stop the session before your dog indicates that he has had enough.
It can be extremely tempting to keep working to see how far you can go but if you can work below the threshold at which your pet has to react you will gain his trust more quickly and help him to learn that grooming can be a pleasant experience.