I have a custom ribbon that I have been using via a PowerPoint addIn.

If I load powerpoint directly then my ribbon appears and all works as intended. If I load powerpoint via an existing file, then the ribbon does not appear although I can see that the addIn has been loaded in the background. Opening an existing file after hacing opened a blank presentation also works fine.

Does anyone have any ideas for the cause of this behaviour? It's only just started happening and the only change I can see is that I am now using it in O365. My Excel version of the same code still works fine.