We're wondering if this is due to preprocessing in the file. When you open the file, look in the Document menu. Do you see an entry that says, "C/C++ Options"? Also, open the Defs tool window. Are all of the symbols defined there?
If the file does contain #if sections, you will need to define the preprocessing values to SlickEdit, otherwise the parser will not handle it correctly. There are instructions for how to do that in the User Guide, under Language Specific Editing > C and C++ > C/C++ Preprocessing.