Probably water is the issue - when you transfer a plant to the ground after so long, it will have a pretty solid rootball, and takes some time to adapt to its new situation, because it now has to find water from the surrounding soil rather than you coming along and giving it some directly to the rootball in the pot.

If its hot/warm and dry or windy, water every other day with at least a couple of gallons all around the base, or leave the hose trickling at the base for half an hour, moving it from one side to the other after 15 minutes. I'm afraid you will have to continue doing this until late autumn, unless wet weather sets in, and probably throughout spring and summer next year when necessary. Its much harder to keep a large, solid rootball moist in the ground than it is in a pot - it requires more water for one thing.

The other factor is that mature plants (which yours is now) often do not settle in and grow as well as younger plants, and that, over time, might be an issue, although there's nothing you can do about that other than keeping its water supplies sufficient and regular enough.