Horizontal line shaking

I have a Monport 100 watt laser cutting machine with autofocus function. Recently, I have encountered a problem where the horizontal line shakes while the vertical line is very straight. I searched online and most of the answers said it might be because the Y-axis belt is loose or the belt is missing teeth. I bought this machine in January this year and used it for about 80 hours. But the belt is not loose and there are no missing teeth.
There is a slight issue with the autofocus function, but this issue only occurs in the X-axis and Y-axis directions, so I believe this should not be the root cause of the problem.
any suggestion？