It sounds to me like the [header] file containing the symbol is not in your workspace, therefore, it is not included in the tag file.
A good way to track down these issues to to use the Symbols tool window to drill down and see if the symbol is in your workspace tag file or not. If the symbol doesn't show up in the Symbols tool window, but when you open the file, it shows up in the Defs tool window, then either your tag file is out of date, or you do not have that file in your workspace.
Note, this is a common problem with Visual Studio solutions, because people set up the solution just so that they can build and do not add header files to the solution.
Another common problem area is extensionless C++ header files. If you have not configured the extensionless header file stuff, SlickEdit might not see the file as C++ and it will not be tagged as you expect.