What version of Windows are you using and what version of Start11?
If you are using the Windows 11 dev/beta insider release please contact Microsoft for technical assistance as their most recent build has broken the show hidden system tray icon feature of their taskbar. This is the case with or without Start11.
There is a known issue on those versions that due to the above issue all the system tray icons do not appear when using the enhanced taskbar feature of Start11 but this can by bypassed by disabling the enhanced taskbar features in Start11 on the taskbar page. This defaults to off however so you would have had to enable it.
Please also be aware that we do not offer any formal support for insider releases of Windows for reasons like this.