I'm using MicroCore 3.0 with the core elements, wbar and jwm. I find that the icons for the sylpheed.tcz and filezilla.tcz extensions do not appear in my wbar.
Both apps appear in the menu and both actually work.
Icons for other apps (emelfm2, minefield, vlc) are ok.
These icons were visible using MicroCore 2.11.5 with core elements, wbar and jwm.