Hmm, this is definitely strange, because it’s not only ‘last successful weather call’ is falling behind, but also ‘last weather call’ is falling behind. This means the firmware is not even making the requests.
Looking through the code (https://github.com/OpenSprinkler/OpenSprinklerGen2/blob/master/main.cpp#L579), there are a few conditions where the controller would not send the weather call:
1) a program is running currently (but no program can run for longer than 18 hours, so this is unlikely the condition).
2) the last successfully received weather call is more than 24 hours ago, in which case it will trigger a network re-connection instead of making further weather calls. I just realized this is actually a bug, since once this condition happens, it will never make a weather call again. However, for this to happen in the first place, the controller has not successfully received any weather call (which was sent every 30 minutes) for 24 hours, and this is certainly an unusual situation. In any case, this is definitely a bug that should be fixed.
For now I suggest you restart RPi or at last restart the firmware.