DIY kit Mk2 - cut size issue

Hi,
just got my first Laser cutter, DIY kit 10w mk2 last week. Have been messing around but have reached an impasse. When I cut a square, the left to right measurement is correct but the top to bottom is nearly a full mm off, ie 9.01mm when it should be 10mm If I change the kerf to make it 10mm the other dimension is too then big.
I need to get this accurate as I will be cutting and engraving small objects.
I’m using light burn over WiFi to control the laser.
Any ideas would be gratefully received

Thanks in Advance
Fastpud