Precision issue with Workbee 1015

Hello,
I'm new to cnc world with a Workbee 1015 (belt driven on X & Y / lead screw on X), Black box (I use OB Cam & Control) and OB xyz probe.
I finished the assembly and tried to do some cutting test but the result is not as expected.
I had an issue with precision and I don't know what to correct to solve it (software and/or hardware).
As you can see on those pictures I just tried to do a 100mm circle and a 100mm square on mdf (I use a 2mm bit) but I can't get the good size and shape.

Here is what I already checked :
- Y axes are parallel.
- I calibrated all the axes with OB calibration wizard (many times).
- The router is perpendicular "as possible" with the spoilerboard.

What will be your advices to improve precision ?

Thank you.

 

Thanks for these leads, i will check this and tell you if it gets better.

 

Hello,
Sorry for the long time reply (health issue).
It was the grub screw on pulley and fine tune on X axis.

Thanks.