@smallfavor where to begin....it is very tough to figure out what you're plan is with this design. if you create a component within the wrong active component take the time to undo and make the correct component active.
the reason the sliver is there is because the bodies/components are in fact not derived for the same sketch. the reality is that there are two unconstrained sketches and therefore not the same.
your file is attached
If you want to use one sketch.....
- make sure it is constrained fully!!!!!
- when you extrude select new component instead of new body each time and make sure the correct component is active before you create it
I played with your model and got it to work but to be honest I would scrape this and start over.
Things I have noticed
- Your sketches are not fully constrained. this is a MUST!!!!!!!
- you also have bodies in the assembly and not in the sub-component. see picture the Body in Slats assembly looks like the component Lower in the frame assembly. BAD idea.
- there is no need for you to be using the move command so much. instead use joints to assembly things
- you should NOT be using mirror when there are no left/right features. use joints here too.
- no need to be using all the combinesust

Timm
Engineer, Maker
System: Aorus X3 Plus V3, Windows 10
Plymouth Michigan, USA
Owner TMC Engineering