Mail Merge templates not showing in the Lookup Template list


I have created some new Mail Merge Templates and turned on Mail Merge in the General Entity Options tab for a number of new Entities within our CRM system. For some reason in a couple of these entities I am unable to see any templates to select in the template list when trying to create a new Mail Merge document from them during testing.

The Mail Merge Button is showing on the ribbon as I would expect in the entity form, but when the Lookup Record form opens after selecting the button there are no templates to select, not even the default system templates. I can see my new templates and the system templates from other entities where Mail Merge has been turned on so there doesn't seem to be anything wrong with the view.

I would be really grateful if someone could let me know how to solve this problem or tell me why it is happening so I can figure out a work around as this has had me stumped for the last week.

Regards Rick