Well, let's get some things out of the way first.

1: Where's the pH level at? Blueberries like the pH to be in between 4.5 and 5.5, but they can tolerate up to 6.5 at the highest. This will limit production and growth, which might be the problem.

2: How often do you fertilize? Do you use mulch? Potted blueberries should be mulched because they like lots of organic matter in the soil and mulching heavily will do that for you.

I think the unhappy one might be root bound, because the repotted one is doing much better than the other one. I would try repotting in a larger container if possible and be sure to add lots of organic matter to your potting soil

I've had three varieties in 15 gal pots since 2004 without repotting that give bumper crops every year. In fact I have to remove about half the fruit. I'd estimate yield at 5-10 lbs per plant.

So you don't need bigger pots. They don't need repotting very often. And I don't mulch the surface. But I do fertilize about once a month and pH is 4.5 to 5.

You probably need a new mix high in sphagnum peat moss. I add enough coarse compost and fine pine bark that the peat is easier to wet. So my mix feels loose wet or dry and holds lots of air. It is about 98% organic and 2% soil. I'd doesn't settle much over many years. The surface becomes a solid mat of fine roots.

Others have different methods that work. But my plants are very vigorous and productive.