MLAT timing errors are usually from three sources.
The location setting is not precise to the location of the antenna. Check your ADSB stats page and see if the marker is in the correct spot.
Your receiver is dropping information and so the timing is off. We found this usually happens with some cheap USB extension cables and recommend trying another USB cable or just plugging the dongle directly into the raspberry pi.
Lastly, the clock on the dongle will change with temperature. We found this happens when the dongle goes from very hot to very cold (dawn and sunset). This effect is most noticeable when the dongle is outside.
The MLAT system will periodically check the timing. So even if the timing is mismatched and MLAT is deactivated it will automatically turn back on when the timing improves.
There is no alert system when MLAT is not working.