I've had a similar issue in mine and it's not an issue with the unit. I haven't really tracked it down except that I know that, like you, when I'm not on shore power for a while I progressively 'lose' amps. I have two banks wired in parallel but the unit really only monitors what goes in and out of one unit. Charging the other bank directly still adds amps into the side being monitored but I think that there are some things that are wired directly to the other bank that 'consume' amps that don't get counted by the bank that is being monitored. I think (thought?) that since they were wired as one it should show both ways but it's not the case. When I start to see that my bank is showing only 90-95% charged but won't accept more than a few amps of charge from either shore power or engines or solar panels, then I just reset it as 100% and wait til I 'lose' em again. OR, I spend several days on shore power where it all seems to 'come back'.

It could also have something to do with the solar panels putting something in that isn't being counted but they typically won't charge much if I'm basically topped up with shore power or engines. I'd be interested in hearing what others have to say as well.