Your comments

I have experienced this problem for a long time. 
I am using a Mac and running Chrome.

It seems like you have to click a very tiny spot within the + box to make it work.
This is a very needed request and @Rob - it will solve your need to get out of Pivotal. 
I'm glad it's under review - I hope it gets picked soon.
For those others waiting for this to be released - What I've been doing in the meantime is establish sprints that are resource focused. While I can't assign the resource in the software to a sprint or story, I can group stories into a sprint, then tell a resource they are responsible for that sprint. It's worked well for small projects and teams, so far, but this new function will be very welcome.
I have been using google docs with a pointer as a solution, but it presents a problem when my client doesn't have a google account. Having a consistent location would be good. I could see and understand where this would present a space problem, maybe space constraints on the item?