Check and make sure that the particular KOBO ereader is supported by ADE.
There's access to the list of supported devices in ADE's Help function.
Some new devices aren't.
Also, did you register the ereader with KOBO?
The Kobo reader is supported by ADE and I have been able to authorise it with ADE. Yes I have registered with Kobo.
The books concerned are showing in downloads as ACSM files and wont convert to Epub
Would it cause problems if I backed up mylibrary to SD card then removed digital editions and reinstalled?
This isn't the kind of problem that reinstalling is going to help. Since
you've been able to download ebooks before, but can't do it with these
ebooks, then I'm 'assuming' the issues are with the ebooks themselves.
I've seen situations like this where the source location has messed up
their protocol, and/or expect the software you're using to be something
other than ADE (usually Overdrive), and that will cause the issues. I'd
start by contacting the source's technical support function and explain to
them what's happening. It could be a quick fix - but it's not something
that you can fix.