Rectangle getting created when solid get sliced

So I’m using dynamo to slice my solids at specific points. but it doesn’t exactly what I want it to do. For some solids it creates a big rectangle instead of just slicing it into two parts, what can be the reason of this? Please note that this also happens when I slice the solids manually.
Now phase:

Wanted result:

Now achieved result:

note: I know that this isn’t totally dynamo related but I don’t know where else to ask it.
Edit: the error I recieve when I do it manualy is: “Slicing plane does not intersect 1 selected solid.”, this is probally the same reason why it happens when you run it.

No one got a clue how to solve this issue?