Asking for advice on trimming the edges. For reference, I'm using a home-built book plough that seems to be producing smooth edges, just not square edges. 

As hard as I work to keep things square, inserting the text block into the press with the support board, and leveling everything? It seems part of the block inevitably sags, or the whole thing gets twisted along the book thickness axis somehow, and I'm getting crooked edges.

In this example (first from-scratch binding afternsome rebinds, I know there are issues) the text block wasn't trimmed very well, but I ran out of page size to trim much further. It originally had a more significant slant, but after pressing and syanding/opening the book to dry it (as DAS recommends, supported text block with some board), it seems the pages evened out, transferring the slant to the cover.

I could also be trimming better than I think, but losing squareness in backing or some other way I can't imagine.

Thanks for any advice people are willing to offer. Unvarnished truth is preferred. I included pics of my home made plough. I think it should be fine (it trims very evenly and smoothly so long as I keep the lade sharp), but of course I lack the training and experience to actually know. Pics #4 is an attempt to photograph the degree of flatness and evenness of the oak beams that serve as the jaws and horizontal plough guide. They seem fairly flat according to my surface plate, but wood is tricky. 

Trimming edges? Other issue?