in the settings of the taskbar that can be accessed after unlocking widgets in the taskbar by making right click/panel options/panel settings you will find a button called more settings
the button called maximize panel should do what you want, also be sure to keep the panel centered, sometimes when is set to left it behaves weird but maybe iti is just because i have it set to autohide
remember to lock again
if this doesn't help, you can delete the whole panel and recreate it, is very simple to do, just follow the previous steps and in the more settings click on remove this panel and accept
to create another you can go to desktop/right click/add panel/default panel
this creates a pannel ready with the basic configuration
you can also add a empty one and add widgets the same way i described