This morning we realised that some days ago our gravity forms had stopped being sent (inFocus theme). The forms themselves appeared to work fine, but the recipient wouldn't get them & the entries weren't being logged.
The solution we found after trying out all we could think of, (including switching to SMTP), was
1) update the plugin (which of course we had done, but step 2 is dependant on it)
& 2) switch on, save, switch off, save.. the Settings > 'No-Conflict Mode'.
No idea what was going on here, but this worked. And even more bizarrely, we got a backlog of submission attempts thereafter... which also landed in the 'entries'. How could these have been saved somewhere?!
The gravity form plug-in on another site without the inFocus theme continued to work fine. So it appears this was a theme specific bug. But better to record it here in case someone else comes across it as well.