I'm going to assume that both plants get equal amounts of sun. fertilizer and water.
Is it getting root bound? Or perhaps the soil it's in has degraded, so it's not draining properly.
Also, since the containers plants come in are normally black, it could be that the roots are getting hot and the soils drying up faster than the one you've potted up. In that case, it's not getting enough water.
I'd repot it with good potting mix, water when the soil is dry and see if it doesn't recover.
barb