Make sure the whole tree is the same priority color.
Otherwise you have the wrong priority color for your object. Also try to place the base of your tree (or other object) where the priority lines change.
Have a look at a few of sierra's PIC screens and priority lines, they may help you get a better idea on what to do. ie, each object in the picture (wall, tree, bin, desk) is the same priority color depending on where it is on the screen.
Have a look at the help file that came with AgiStudio, I'm sure it has a section about the priority screen colors.