While attachment behaviors are very powerful, I first suggest you try attaching the line to a snap behavior and see if it works for you. Typically you need several attachment behaviors (often 4) in order to stabilize an item. Snap gives similar effects with a single, often easier to use, behavior.
As a separate issue, you're setting up your collisions incorrectly. Just add all of your views to a collision behavior (using addItem:
). You don't need to create a bunch of boundaries.