There are not many problems with IoTaWatt these days. What few issues are reported are almost all SDcard or poor WiFi. There was a spate of SDcard failures due to a few hundred defective cards, but none of the units you bought from the stuff shop are subject to that.
I’d look at WiFi first. RSSI signal strength is not the only indicator of WiFi reliability. The unit could be using a different AP on a different channel from the others or there are a lot of devices that use the 2.4GHz frequency, including wireless phones. A poor quality device close to your unit can cause WiFi problems.
If you are certain the WiFi is not a problem, the next step is to replece the SDcard. If you want to try to keep your history, I would recommend copying the /iotawatt/histlog.log file. That will retain your 1 minute history and the history log is much less likely to have damage than the current log.