"New parameter" field in custom build dialog gone AWOL?

I have typically added a new parameter at build time for specific builds in a certain project. However, the custom build dialog is no longer showing me the option to define new parameters (I can still see builds in the history where I set the custom parameter, so I know I'm not completely losing my marbles); I can only edit the predefined parameter values and can't for the life of me figure out what's changed to cause that field to be hidden.

No recent upgrades or permissions changes to my account, running 7.1.4 Enterprise. If anyone can point me in the right direction, I'd be grateful.

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