I have a vertical panel with several launcher buttons. When I try to change the position of the buttons, moving just fails.
The code for this heavily changed in 4.7 and the upcoming 4.8. Maybe you can give the latest 4.7 development release of xfce4-panel a shot?
FYI: 4.7 also allows to reorder plugins in the panel preferences dialog with up and down buttons.
Uh, did this happen after a gtk update (you gtk version is 2.22) and also occurs with a horizontal panel? In that case this is a duplicate of bug #6818.
I think, I installed Xfce, when GTK version was already 2.22 (in Arch Linux). The horizontal panels don't have the same problem as vertical panels. I will probably try 4.8, when it's released.
If this is the Gtk bug moving works if you start a plugin move and then only a drop zone is highlighted when you drag over the last 1px border of the panel, not over other plugins.
I am not sure, if I understood your comment. If right click on an applet in a horizontal panel and move the applet to a new location it really moves. In case of vertical panel there is no "black cursor" moving in the panel. I tried to move the gadget over the last pixel column of the panel, but it didn't solve anything and broke auto hide...
Well this code changed a lot in the 4.7 panel. So closing this bugs since there won't be and 4.6 panel releases. Might also be somewhat related to bug #6818.