There are two types of insert

A single piece one which as it is tightened will split into two as the nut is tightened and then grip the pipe.

The other part has one piece that is placed into the 22mm socket, then a 15mm olive then the second part of the reducer goes on followed by the nut.

It may be you have left the olive out of the later or not tightened the singel part one enough.

A little jointing compound will help the nut turn without gauling, if you dont have any try a drop of vegitable oil.

Jason