Do you think if the spring tension in the cartridges isnt' quite right then it could struggle to keep up with the motor.. I've had a couple of snags at very shallow depths and figured that if the spring couldn't retract fast enough then it would cause the needles to not retract fully. Just a thought.

After this happening a couple of times I tried lining with the machine angled forwards slightly rather than straight up and down and it seems to produce a much nicer line.