Mush in the Middle

Why do the middles of books, like peanut butter and jelly sandwiches, tend to get so mushy, squished and icky?

I am trying to brush up my book and I think the beginning and ending are adequate, but Chapters 12 and 13 are simply undigestible. I honestly don’t know what to do with them.

Some of the problems are:

* too many truncated scenes giving a staccato feel to the chapters
* low tension sub-plots
* time bridges
* scenes which serve to set up later tension but are otherwise boring
* merely cutting or combining scenes results in illogical sequencing