Add a motor slipping ?

[Aquaman05]

Installed my add a motor last night and i think it is slipping on the way up.
About half way up it slips once then continues up the rest of the way.
Anyone else have this problem ?
My string is straight and my door is a no parking sign 12 x 18.

 

[KDK1]

Just a thought, but it sounds like the the line is jumping off the rubber pulley. Mine did that during the "fine tuning" process but then again, I used the drapery motor and rigged it for a door.

 

[Aquaman05]

I'm using the add a motor d20, it don't have a rubber pully I can see.