WFCO converter causing generator to rev up and down

Hello I'm new here. I set up an off-grid system over a year ago and it's been working great. I recently got a backup WFCO converter in case the old one breaks down. Both are 55 amp wfco converters. The old one works perfectly with my generators. The new one causes the generators to rev up and down wildly as it appears the converter is cycling between absorption and bulk charge. I can get it to stop by turning the RPM screw on the generators down so low the load turns off and then turn the rpms back up. As soon as the voltage at the batteries hits 13.6 it starts the cycling again without fully charging the batteries. The only thing I've been able to find on this was possibly a "shorted battery cell and other problems". I'm interested in what these other problems might be and any idea someone here might have.