Dogs are pack animals. The primary social factor affecting a dog's life in the pack is the dog's status in the pack. When a dog is on its back, in the dog world, this is an act of submission. (Because it's neck is exposed and the dog on top could kill it if it wanted.)
That being said, it's hard to tell if there is a problem without more information. In other words, your dog could be an alpha-type that instinctively rejects doing acts of submission (like being on its back).
If your dog thinks its the alpha, however, this will eventually become a problem as it will expect you to do the submitting and could potentially bite you in order to try to enforce its dominance in the pack.
No, a trainer can not correct it. As the dog will perceive the trainer to be the alpha. Which will still leave you with your problem of appearing to your dog as being lower in pack status.
You might want to consider studying on this topic. Even if your dog is at the extreme end of the alpha personality spectrum, you still hold the trump card: you control the food. Properly managed, you could use that factor alone (your dog's sole source of food) if necessary, to secure alpha status with your dog.
As another answer mentions, if you use a trainer, get a human trainer (to train the human, i.e., you) not a dog trainer.