Chinese laser cutter issue

[Williamel]

Hi guys,

I have had my ~ $800 chinese laser cutter with moshi bought on ebay for about a year.
recently i have noticed that the cuts are not clean.

Please take a look at the attached file (the material is printer paper):

1. the circle has a little bump in it
2. the square is not a square (3 sides are perpendicular but the right one is not)

Both shapes are around 1/2 inch in dimension

Any advice where to start troubleshooting it ?
I am trying to come up with a hypothesis - what could cause such a weird and repeatable deviation ?

any ideas would be very much appreciated,
Regards,
Lenny