Without having been to that station myself, it’s hard for me to guess why that station is having more battery trouble than your other ones. I’m guessing maybe it’s more shaded, or possibly the solar panel isn’t pointed in an optimal direction? You could also try putting a voltmeter on the panel on a sunny day and make sure it’s putting out around 6v (or even better, put an ammeter in line and read the charging current). We’ve had a couple of panels fail over time and don’t provide enough power to fully charge a battery. We’ve also seen issues where the charge controller circuitry on the Mayfly would partially fail and cause it to default to the trickle charge rate instead of full current charge. We’ve also had malfunctioning GPRSbee modules that tend to drain the battery power quicker even when asleep. I’ve also seen problems with microSD cards that start to go bad and draw excess current when the Mayfly is supposed to be sleeping.
So I’d say check your solar panel voltage, remove your GPRSbee, and replace the microSD card. If none of those fix the issue, put a new Mayfly in there. If the battery levels still fall, then replace the solar panel. If none of that solves the problem, then it’s likely just a shady location. If you’ve got deciduous trees around the station, sunlight should be improving in the coming weeks as the leaves fall, so that may also affect your results. We have a few stations that always struggle to stay charged during summer because of the full canopy above them, but they do great the rest of the year, even in the winter when the sun angle is lower.