Migrate from Access 2007 to 2013

I created an in-house Access front-end many years ago to manage our order book which has been working faultlessly ever since. It has a remote MySQL back-end where the tables are import-linked back to the front-end. Again no working issues whatsoever.

Now we are moving from XP to Win7 machines and Access 2013 (Office 365)
In my Access front-end, one part has a simple form / sub-form set-up.

On Win XP We enter new records into the sub-form without a hitch, in Access 2013 all I get is the 'Write Conflict' error when trying to add a record ??
Also, in Access 2013, the button I have to delete one of the records just does nothing and cancels the event. I even tried using the button wizard in 2013 to create a new delete record button and yet still, I just get a message, event cancelled.

In other areas in my app where we enter / edit / delete records it works fine in 2013, just in one particular form / sub-form set up.

Works in Access 2007 on xp, not in 2013 on win 7
We really need to migrate over to the new pcs, but unless I can get this one area fixed I can't and it's really holding us back.
I cannot see any obvious reason why it would throw up these probems.
Hope you can help