Promapp purposely doesn't allow you to loop back as that generally indicates and exception to the process (ie something has gone wrong and rework is required).
One of the main principles applied in creating process maps in Promapp is to simplify the process and to map the flow of the process that occurs 80% of the time; to not clutter up/obscure that view by including all of the possible exceptions and variations.
There are two articles in the Promapp blog detailing how to reflect exceptions in Promapp that I've found helpful:
Managing-process-exceptions-in-Promapp
Decision diamonds
(not a Nintex employee, just a big fan of the principle of simplifying the way process flows are expressed.)