Are any of the percussion instruments bracketed together? If not, no system objects will appear there (as explained in the text at the top of the System Object Positions section of Layout Options).
If it’s not that, we’ll probably need to see a project; this isn’t a common complaint so it’s not possible to diagnose from a few lines of text.
side note: I’d like to see this restriction on System Object Positions relaxed. In the case of percussion in particular, brackets can vary from system to system and it’s disorientating for one page to have system objects at three locations and the next to show them only at two locations. Not to mention (in-line) piano duet music, where it can be useful to system text above both grand staves.