i HAVE THIS SAME MACHINE ONLY SOLUTION IS THE OFFSET THATS THE ONLY REASON YOU WOULD HAVE TAILS GO TO GRAPHTECS WEBSITE YOU CAN GET THE MANUEL FROM THERE IF YOU DONT KNOW HOW TO ADJUST THE OFFSET IF YOU CANT FIND IT OR DONT KNOW HOW PM ME AND i WILL GIVE YOU DETAILED INFO ON HOW TO DO THAT I CAN SEND YOU THE MANUEL IN ODF FORMATT IF YOU CANT FIND IT.\ HOPE THIS HELPS
Stop YELLING and learn to use a keyboard like a civilized being.
While this problem might be offset, the OP said that it seems to be getting worse over time. Mis-setting the offset won't get worse, it is what it is. If this is the case, that it's getting worse, then it's probably not the offset setting.
It could well be a dull blade. Change that first. With plotters, if you even think a blade is dull, it is. Change it.
More often than not, creeping over-cutting and in general, creeping anything, is a symptom of the Z, that's up and down, positioning mechanism not being able to move as freely as it should. Or just wear and tear on the carriage has put it out of spec in some way or another. Before calling for the Marines, try removing the dust cover from your tool carriage and blowing the dust and crud out of it while gently moving the Z mechanism up and down. More often than not this will do the trick. Also, when was the last time you put just a speck of silicone grease on the tiny ball bearings at the top of the blade holder?
More obscurely, I might suspect some deterioration of Y belt tightness and/or X and Y drive precision.
Offset is to plotters what carburetors used to be to cars. It's the first thing everyone jumps on in time of trouble and it's usually that last thing that needs any attention. But it's big, it's right there, and it's poorly understood.