So, I have fixed my error. It has nothing to do with the spreasheet recalculation (for those of you who are interested).
My form is for staff reimbursements where staff must upload supporting documentation.
In the Autocrat job the upload field is set as a hyperlink so the person authorising the claim can see the supporting documentation in drive.
The only difference with the line that 'hung' and others was that it had 3 uploads, all the others had 1.
I deleted the hyperlink and the job executed perfectly.
To work around this if I want to keep the hyperlinks I will have to add an upload option at the end of each question instead of one 'bulk' one at the end of the form.
Hope this helps.