Trying to cut a shelf but keeps messing up!

Hi, I’m trying to diagonally cut the bottom section off of my shelf with the lines that are to the right of the shelf, I’ve tried wirecuts and cutplanes but it keeps doing it wrong, please help! Thanks.
Apparently file too large :frowning:

Hi James- please Export the objects you are working with and post the file here or to if it is confidential.