you can adjust that by clicking on the "File" ribbon, there you select "Model Settings" and in the tab "3D" you have to set the "Screen coverage threshold" to zero.
The MUs probably have multiple level(s) of detail (LOD).
Maybe you can delete the ones which are not fully displaying all MU details
Have a look at the 3d structure of your class MU and check and then delete the LOD.
Backup/ Duplicate the Mu before you start deleting any LOD !
what you can do in this case is to open the 3D Graphic of the class of your custom object in a new 3D window. For that you right click on your custom object in the Class Library and choose "Open in 3D". There you right click on it and choose "Optimize". Here you click on "Preview Result" and if the result is acceptable afterwards on "Accept Result". Afterwards this problem should be fixed, but if that issue is still persisting could you post your 3D object here?