Print dosent stay attached to suppots

[CollinsA]

I have been testing this little printer for the past couple of days trying to get proper sized parts. When I place the part on the platform and print I have a noticeable short dimension in Z. For example a 20mm cube will come out 18mm in Z. So I thought I would try and build off the platform and use supports, I have had zero luck. Each time I build the part breaks or falls off the supports during the build. The supports are printed fine and are still on the platform but my part is partially built and is down in the resin tray. I have tried all but the heavy supports. I'm still trying to just print off the 20mm cube, I thought I would rotate the cube 45 degrees so the bottom would not be parallel to the platform, then I rotated my cube to where one corner was toward the platform- these all failed with the supports being built and a portion of the part n the resin tray. Any ideas?