Hey,
So i’m trying decimation on a Ztool. It worked correctly for a few subtools, but for a few others it instead triangulates the mesh at full resolution.
If it helps at all I noticed when preprocessing it only lasts a second, while on the meshes that were successfully decimated the preprocessing lasted longer.
Any help would be appreciated! Thanks 
Could you make sure you have the most current release of Decimation Master? An update was released a few weeks ago, on the Plugin page of the Download Center. See my signature.
This problem can happen if you did an AUVtile on your model and then having the Keep UV option one.
Or perhaps you have a mask which visibility is off.
Thanks guys I did both and something worked cheers!