That is exactly what will happen if you try to delete a part of a single track in a multichannel file. If you think about it, that makes sense; if you removed the time then you'd be removing it from the other channels too. So all that can happen when a single channel is selected and the delete key pressed is that the content is deleted, not the time.
The solution is to split the multichannel file out into its component parts, and edit those independently. Then the delete key will operate as normal.