Icons disappear from the panel

[jwegier]

it seems that something strange happened after I moved some widgets on the panel from side to side and then back to their original places. I suddenly noticed that the icons of opened applications stopped to appear on the panel on the right of the "M" of Maui logo/menu launcher

That's how it looks now:

   

In spite of Google Chrome being opened its icon does not appear next to the Maui logo on the panel. Other applications are also opened but their icons also do not appear on the panel next to the M logo

Thanks