I also had one in a 15 gallon, it wasn't doing too well so I took a cutting and grafted it to an in ground cherimoya. The one in the pot died, and the graft has flowered the last two years and sets fruit late around now, but I haven't got a fruit to survive to maturity. I don't get frost, but the prolonged under 50-55 degree nights and not really any warm days for a few months is probably the reason it doesn't thrive.