Imp is always in range of the boss and therefore it won't move.
You keep moving and therefore get more than 40 yards away from Ímp.
Since Flee only has 40 yard range you are out of range and hence it won't cast.
Since Imp is out of range you can't dispel yourself.
You will probably notice that once you enter phase 2 and attack a new target Imp will come towards you (assuming you have it on assist) and you can once again dispel.
So in order to fix this you would have to use Flee on CD and even then it may get out of range.
Alternatively you can use the 'Move to position and stay' function and place it in the area you are heading during phase 1.
Locks need to help dispel bc 2 healers per side and sometimes one side will get 3 bombs
I was helping last night. The best way I found was placing the imp b4 the fight in a middle area where he could reach people in P1 and P2. It's still not perfect and you might have to reposition
- - - Updated - - -
I'm toying with taking Sac talent and sacrificing imp to make dispelling easier. Would lose haunt though