Volume envelope problem

Is this an X1 feature that I fail to appreciate or a bug?  I can't get volume envelope tool work...sometimes. Very frustrating.  I highlight the clip, all nodes turn green, put smart tool at top of clip, click to drag nodes and all nodes disengage and do not move.  very frustrating.  It's happened 3 times now with no rhyme or reason that I can detect.  Am I causing this or is it a bug? I have X1b installed and not 255, was this addressed in 255? 

Hi Scott, thanks for the tip. I just gave it a try and I'm thinkin I  don't have a surgeons touch because there is a lot of lateral envelope movement when I grab a selected node. If I drag from the top it creates two nodes in the area I am working on and allows me to only move the envelope vertically.  It is odd though it usually works just fine but on 3 occasions it just wouldn't function.   There was plenty of room between my volume level and the top of the clip.  Normally I wouldn't even worry about writing in but  I had so many volume changes on this track that, short of adjusting the gain, moving the whole envelope was my only option for fine tuning it.