More than likely the alignment of the silencer to the flight path of the pellet from the barrel is off and the pellets are clipping either part of the silencer internals or the exit hole of the mod. Give the mod a shake and if it rattles or lead bits fall out then you know for certain it's shaving your ammo. If this is not the case then still have a look at the exit hole anyway and see if you can see signs of any impacts, or fire a pellet into a medium which won't damage the shot pellet and inspect the expelled pellet for damage.
When you have determined that it is clipping you will then need to find out why - most likely is that the threads on the barrel/adapter aren't true, or that the adapter isn't fitted correctly, or that the bore is not central to the barrel, or there might be the possibility that you are using a .177 silencer on a .22 barrel.