Why can't I add a Design Flow Rate to my Space Type?

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Edited to add - looks like a Windows 10 problem

I'm trying to add a Space Infiltration Design Flow Rate for my building. I have a number of ACH rates set up in a library which I've imported but when I'm on the Space Types > General tab I'm not able to drag them into place. In fact I don't seem to be able to drag anything into place anywhere.

It was working previously but I'd like to avoid going back to an old model if possible. What could have gone wrong with this model and is there any way to fix it?