Bougainvilleas can definitely be grown in pots with good results. If they are overwintering indoors, don't expect blossoms, but otherwise, as long as you are pinching to encourage branching and giving them at least 5 hours of sun a day, they should be fine. Potted plants also need regular feeding, much more so than plants planted in the ground. They also have to have really good drainage. The problem could also be in the source for your Bougies. There is truth to the fact that some plants are better than others. How they are started can be just as important as how they are cared for once you get the home. I hope this helps.

Here is a link that might be useful: How to Grow Bougainvillea

In a pot a plant can only grow as much as the space provided for roots in the pot. The general rule of thumb is to pot up an inch or two every few years, if your goal is to increase the overall size of the plant. If bougies are fast enough (like wisteria), then you can pot up at larger increments. I plopped a wistera from a 1gal to a 5 gal and filled it in one season.

Decreased growth, or vigor, is typically a sign of root congestion. If you take a look at the root ball, and roots are circling everywhere and lifting all dirt....it's time for some root maintenance. I don't know when it's best to root prune bougie's though.