Not sure whether you are running 12 or 24v but I still suggest you test each unit on its own, Yes of course it could be a connection problem, you may be lucky looking for volt drops across the connections with a meter.
Looks to be a good grade battery but age is a killer as well as number of cycles of discharge. If you use them they wear out, if just left on float they eventually die, can't win with batteries.
See what you can find out next time you are there, may have a better idea of things to try with more information.
Flux