I have a couple of elderly-but-good Nexus7 tablets which have been working well for some time but currently refuse to play with my Skyecho. 

It seems that the latest tools that we use for building SkyDemon for Android have resulted in the loss of this functionality on those particular tablets or that particular old operating system. There isn't anything we can do about this, however to keep your old tablets going with SkyDemon you can revert to an older version of SkyDemon (available on our website) if you wish.