Hi - I've had a search around for an answer but can't find one that specifically addresses my issue!
I have several Lists in a 2013 SP site and one of them has the "Nintex Forms" icon greyed out (in the "Customize List" part of the ribbon).
I have checked the List Type and it is 120 - Custom grid and not 100 - general. I'm thinking this may be why. The list has a lot of data and calculated columns in it. If this is the reason, is there any way to change the list type?
are there other lists on the same site that have the button enabled? or is the problem site wide? could you try deactivating and reactivating the feature?
Hi Cassy, all the other lists are on the same site, so it seems to be an issue specific to that list. I recreated the list from scratch and the nintex form is available. To add to my Nintex woes, I've created a workflow on the new list and, when I click "edit start form", nothing happens. One of those days.
if you are still unsure how to go about the issue you are having, I would suggest for you to raise a Support Ticket to Nintex to assist on this issue. 
refer to this post:Button "Nintex forms" is disabled for lists which are imported by MS Access
Thanks Mark Cooley! It works.
As a workaround, I have managed to successfully create/edit a Nintex form for my list by navigating directly to http://siteurl/_layouts/15/NintexForms/ListFormDesigner.aspx?List={ListGUID}
ListGUID can be obtained by editing the list settings and copying it from the address bar
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