Ahhhh. I remember this frustration.
The secret is in the name. Static COLlection. i.e. static objects. If you have any other object type, the CK does weird things if you try to SCOL them. I have seen some pretty crazy graphical effects when creating a static out of some of them.
There is one more important fact to this. If you make a nif from a SCOL (new static) and try to add that to the SCOL you will encounter the same thing. I went through a lot of this. Most of the
clutter statics from SS2 are made from SCOLs. It took some questions on the SS2 addon author's discord to figure this out. You may want to request to join.
There usually someone around that will have an answer for beginner questions. Once you get more to the advanced level, its mostly by trial and error (mostly error) to figure things out.
The moral of the story is, if you can't make a SCOL, double check all selected objects and make sure they are static. If you are using any SS2 custom assets, there is a chance it was made from a SCOL and can not be used to make a new one.