Zbrush not decimating

Hi , I have a 48 million active point mesh , wich is a sort of cliff, so I painted in some polygroups and split the meshinto several subtools to decimate freezing borders... I pick one subtool wich is now 6 mil active points and I set decimation to 50% , it starts, it does he preprocess and then the decimation , but the active points remain unaltered ... why ? What's wrong wich Decimation master?