I assume you are doing this to for a test to see if the maps you have downloaded are going to work when you do get in the field . Sounds like a good idea. I guess you have turned your Wi-Fi off and maybe went to airplane mode to disable your cell service.
I haven't used XMaps but I have played with Backcountry Navigator a little. I have downloaded maps of areas I knew didn't have cell service and wen I went there and hit the GPS the maps came right up.
I have a phone that has a removable micro SD card. Once the phone asked if I wanted to store the maps on the SD card and I said yes and then the phone couldn't find them. I assume it was looking on the internal memory and not the SD card. Haven't figured out how to change that yet. Downloaded the maps to the internal memory and they worked.
So if your phone has an SD card the program could be looking at the internal memory and the maps are on the SD card or vice versa. But if you have an Apple this isn't the problem because they don't have SD cards.
Don't know if any of this helps but let us know how this comes out.