Ooznest OX not cutting circles

Have been trying to cut circles on the OX but they all just keep cutting with 'flat' sides. Have reduced the travel speed of x & y axis but nothing changes. This one was 200mm across, used Sketchup to draw.

Even bigger circles are exactly the same, all have flat sides

 

Will check now, how tight do these need to be, can I 'really pull them'?

 

Thanks, just got round to looking at this. I tightened the belts but this did not make a difference. I then re-adjusted the rollers on the X Axis and hey presto: It's working. They were just too tight.