I had this problem with my Oregon 450, when I made a stripped down set of National Parks maps for myself.
GMapTool’s Details button showed that the National Parks map and the 1:50k map both had a draw priority of 20.
Using the Write tab, I altered this to 25 on the National Parks map. Problem solved!
NB this didn't work with the file in-situ on the Oregon - I had to do it on the PC and reload onto Oregon.
Dales.