To get back to the OPs original statement...

You can infact lose a jig to structure, I personally have done it many times...

Some rock piles ARE structure, as they are part of the natural contour of the land and/or lake, if you get your jig stuck between the rocks, and you lose it, you have lost that jig to structure.

Now MOST times, you do in fact lose it to cover, but not always, just thought I would point that out.

Cheers!

Mike A!