If you are encountering an issue that downloaded offline map data are not shown on main map screen in GPX Viewer PRO this may have several reasons. Solutions that will solve most of the cases of this issue are as follows:
Try zooming out by - button in the bottom right corner as much as possible.
If no map is shown, in top toolbar tap on map button without globe. If you hide this button previously tap on … button in the top right corner of this top toolbar and from the menu select Offline map. If you have also hidden this button, open the left main menu, then tap on the Settings button. In the settings screen tap on the Maps button and then Offline map data manager, when you do this you can continue to step 4 of this how to.
In Offline map dialog tap on Download button at bottom.
In Offline map data manager tap on … button in the top right corner and select Check offline map data integrity, if it will show a dialog that there are problems with offline map data, tap on Yes button to fix them. Then go back to the main map screen and check whether downloaded offline map data are shown. If not, open again Offline map data manager and continue to step 5.
In Offline map data manager tap on + button in bottom right corner and tap on Worldwide low zooms map data. When the download is successful go back to the map screen.
If no map is shown, open Offline map data manager again, then tap on … button in the top right corner and select Delete all offline map data.
If still no map is shown, uninstall GPX Viewer PRO from your device.
Restart your device.
Install GPX Viewer PRO from Google Play Store again.
Try downloading worldwide offline map data and see if it is working.
If the worldwide offline map is working, try downloading other offline map data.