Actually groomers use Furminators to 'card' coats, especially in terriers when done in a proper groom, in conjunction with stripping. Your dog may have an underlying problem causing the hair not to grow back properly, like hypothyroidism. We see this frequently in dogs when something 'different' was done, although the problem may have been there undetected for a while. A poodle that I groom that had a beautiful coat & proper weight went to the vet for a hotspot from a bout with fleas for the first time in her 12 year old life. The hair that they clipped grew back very sparsely and slowly in 12 weeks. She had to beg the vet to check since the dog was atypical for hypothyroidism, but she had it. And that isn't the only disorder that will cause hair not to grow back properly.
But, back to the beginning....groomers do use Furminators to card terriers, although you may have been a lot more vigorous with the tool....who knows....at that age, I'd get it vet checked. The fuzzy body coat is lightly removed with a deshedding blade, and that is what the Furminator is. The purpose is to promote new growth of hard hair with good color. Carding helps to maintain a healthier coat, and done before the stripping, helps the other hair to lie properly. So, you may have done too much, but in reality, if it isn't growing back properly, you pet may have an underlying endocrine problem.