In the ground or in a pot? And if in a pot, how long's it been in the same pot?

They have been in large pots for about a year, the two trees are about 6 ft high

Check the roots in the pots - easiest to turn them out and have a look, I suspect its time for bigger pots - if the roots take up most of the pot, then its difficult for the plant to ever get enough water, cos there's not enough compost to hold it for them. Repot in fresh compost in larger containers, if that's the trouble, and keep well watered. At this time of year, that's most likely to be the problem, assuming you have no other signs of disease or infestation such as scale, etc.

Thanks, I will try larger pots. If I plant them in the ground will they grow very high?

Yes, they make not terribly tall, but incredibly dense trees, making dense, dark shade beneath.

Thanks, I will try larger pots.