Although pears and apples on limiting rootstock are presented as pot plants they would really like to be in the ground. It is a stuggle to get them productive and keep them healthy even when in the ground!! It could be pot bound, starved or or short of water or all three. A small pear that is not thriving may be attacked by pests, rather more than a healthy tree. My weakest pear, and they do vary a lot, has not extended its growth so far, as it has been so dry.