Accuracy getting worse means something is getting loose. Chances are good it is the gantry belt. When you turn on the machine it sets “home” and measures from there. It only measures by keeping track of the turns of the drive wheel. If the distance traveled is different then it will be off by that much.
If any of the belts are slipping, or if the gantry is hitting some sort of debris, then that throws the measurement off.
Turning the machine totally off and then moving gantry and head see if there is resistance anywhere of if holding the belt you can still move the head or gantry, then your issue should become obvious.
If you move the head or gantry while the machine is on then that will mess it up too.