The heads could be the problem.
Over the years while fishing walleyes with a crawler on a # 8 Lindy rig I would on occasion hook a large Gill. They always hit the crawler in the head as to kill the bait and get hooked. In fact one trip up by Orr all we used to catch some dandy Gills was a whole crawler on a Lindy rig. The dinks would always nip at the tail of the crawler while the big fish would head bite.

