And some dogs don't bark much either ! It's all to do with the personality of the dog. Your chi isn't incapable of wagging his tail, he's just not bothered to do it very much. One finds litter sisters and litter brothers who vary like that, just as with other traits One pup is the over-eater, one is the naturally slim one, one goes scatty at many things, wagging his tail so hard it might drop off , another doesn't and so on. Your chi may have a litter brother somewhere who's wagging his tail at everything and anything ! That yours is a rescue dog almost certainly doesn't make any difference in this..It's not as though he's showing some trait which could only be the result of bad experiences, such as being jumpy or cowering. He's just not that demonstrative.