Ribbon Workbench: Display rule works, Enable rule doesn't

I want to show/hide a ribbon button based on the value of a tickbox on the form.  Using the Ribbon Workbench, I've used a Display Value Rule and it works perfectly.  When I replicate the logic exactly using an Enable Value Rule it doesn't work.  I've double-checked that the Rules are linked to the button via Commands correctly.

Any ideas why this might be?

(BTW, the reason I can't just stick with the Display Rule is because this is a test for what I actually want to do, which is call a custom JavaScript function, which you can't do with Display Rules).