battery controller problems, ctek250s dual

Hi to all, I'm after some guidance with regards to my battery controller
I have a CTEK 250s dual which I purchased online and fitted, I have it wired so that it charges the AGM battery from the alternator when in transit and when stationary, charges from folding solar panels via anderson plug.
One morning after not using the camper for a month or so I noticed that the battery voltage was around 12.8 volts, so I plug in the solar panels and notice that the volts climb to around 14 and is charging at about 4 to 5 amps, all good, plenty of direct sun and no shadows should give the battery a good charge over the course of the day.
In the afternoon I went to fold up the solar panels and noticed that the volt gauge was back at 12.8, next day I kept and eye what was happening.
The solar panels starts to charge with 19/20 volts coming out of the solar panel terminal on the controller, 14.2/14.3 volts at the terminal (checked with multimeter) and at 4.3 amps, the solar panel light illuminates and the battery light illuminates and all is well, after about 15 minutes I noticed the volts gauge begin to fall, also the battery light goes out as if the battery not connected, so I unplug the solar panel and wait until the light goes out, put the solar panels back on and all is well for about 15 min and then the same again, so I check to see if the circuit breaker hasn't tripped, all good there.
So, before I start pulling stuff apart does this sound like a battery controller problem or could it be battery.
battery seems to hold 12.8 and run lights and radio with out any problems but I would like some other opinions for possible tests that I can do before I pursue CTEK with a warranty claim on a faulty controller.

Shane