How do I get touch-and-move to work well?

I'm trying to fix my game, Newton's cannon (at newton.zappable.com).

The cannon in Level 2 (and 3) is supposed to move when dragged up and down and that works fine with a mouse. However, it is very difficult to move successfully with a finger (on a phone). Is there a way I can fix this? Thanks!

This is my game-code:

Condition:

Touch - Is touching cannongun

   cannongun - is not overlapping grass

   Action:

   Rotate 10 degrees towards Touch.X, Touch.Y

(If it is overlapping grass, rotate 10 degrees towards 0)

I tried making the cannon much wider, but that doesn't help. In fact, the cannon often goes in the opposite direction of what its supposed to.