There may be a large image, incompatible data in a field, or something wrong with the entry. This is not an issue we have experienced often enough to have a go to reason… We would need more info…
To troubleshoot, try this… Duplicate the entry using Formidable’s “Duplicate” function in the Admin Entries page, and see if it merges, if it does merge, then something’s wrong with that particular Formidable entry.
If it doesn’t work, then somethings wrong with the data.
Try editing the entry and deleting data one field at a time and testing the merge until you find the field causing the merge to fail.
Then, let us know the results… we remain at your service.