You don't say how large/old the tree is, or what your temps are. However, you are most likely underwatering. At ninety degrees, on a mature clump, I would estimate I am using about 100 gallons 3 times a week. Also, if you have one dripper, it will probably not get the water evenly to all the root area (if at all). On a year old plant you need to water a 6-10 foot diameter area.

I think the most important thing in watering bananas is drainage. When I dig a hole to plant a banana, I first fill the hole with water, and see if it drains good. If not, I dig deeper until does.
I have three bananas together, and I water them with about 50 gal. in the morning, and the same in the evening.