[SOLVED] Uploading multiple mapsets to Legend HCx

I am continually frustrated by my Legend HCx not fully displaying mapsets I have uploaded. The content I use is as follows: 1) OSM data downloaded from Geofabrik and processed through mkgmap, incorporating a style file - this data is used for appearance. 2) Roger Calvert’s British National Grid. 3) TalkyToaster’s OSM British Isles - this data is used for routing.
If I upload mapsets 1 and 2, both display. When I upload all three, however, mapset 2 does not display, although it is present. I guess it is hidden below mapset 1 in this instance yet not when uploaded without mapset 3.
As an alternative, I have incorporated a .osm file of the British National Grid with the processing of the split Geofabrik files and uploading the resulting gmapsupp.img with mapset 3, but without success.
I should be grateful for any further suggestions.

I thought I had already tried different draw priorities. It seems not. Giving mapset 1 a draw priority of 20 and mapset 2 a draw priority of 28 allows the grid to display when all three mapsets are uploaded.