Yes, it should lock. My initial thoughts on why you're seeing lines:

1) Ducer is tilting up because it's not locked either producing lines from bad angle or due to air getting under it while it's tilted.

2) With the ducer in place and your motor trimmed down, do you have clearance from the back of the ducer to the foot on your motor if you look accross the transom on the trailer? My motor will interfere if I have it tucked, but then again, only one side of the beam gets intereferance....so that's prolly not your problem.

Therefore, I'm going with number 1.

Let's see pics of that install.

Wannabe...