Tooling split: Can't knit sheets together

I'm currently working on a mold-design product and i'm building a core and cavity with the tooling split. However, I can't manage to get my tooling split to work and the tooling split cannot knit sheets together.
Maybe one of you guys can help me with this problem and explain what i am doing wrong so i can shut down this problem for future parts?
Thank you so much in advance.