The trouble with plum tomatoes is that they are very susceptible to blossom end rot.
Blossom end rot is caused by a calcium deficiency in the plants.
Wild variations in soil moisture causes difficulty for plants to take up calcium.
Pots tend to vary wildly in their soil moisture, unless drip-watered constantly.
As such, Romas are difficult to grow in pots, especially if you live in a hot environment.
Even here in the UK, I'd be reluctant to grow Romas in pots.
It's not impossible, but difficult.
Add some pelleted composted chicken manure to your pots, it's something like 6% calcium. It'll help a bit.