Some apps not in single app mode (Apple MDM)

Hi,

I've implemented an all Apple iPad management solution at my school, incorporating DEP and VPP, with Apple Server and Profile Manager on a Mac Mini running El Capitan, with initial builds done via Apple Configurator 2. All latest versions. Generally very pleased with the management capability of this system. Been running it for about 6 months and, apart from an Apple Configurator update that lost my Blueprints, all ok - building new iPads, rebuilding old iPads, deploying apps and controlling settings. Fine. But now that I come to set groups of iPads into single app mode, I find that some of my VPP purchased (but mostly free) and installed apps are not in the drop-down list of apps to choose from in the Single App Mode "Lock to App" form. I can't see a reason - I thought that it may be an age restriction issue, but doesn't appear so - currently trying to 'single app mode' "Bee-Bot" - 4+ rated. The devices are all supervised and apps are device-assigned, no Apple ID used. Does anyone have any ideas as to why the list is not populated with all the managed and installed apps?

Thanks in advance,

Ian (UK, Primary School)