First thing that comes to mind is you are running 2 main pstems on each mat and you are using 8-10-8 fert. To thrive, bananas require a fert ratio of med-lo-high. How much, say by the cup, are you putting down?
Sounds like they are being starved of potassium.
Two main psems on a mat means higher demand of K. The monthly part is on the nose.