I figured out my version of this problem. I only recently turned on this feature in the last 3 weeks. It was working great until that button just wouldn't appear on a specific client invoice I was trying to email. Turns out this particular invoice I had drafted on a date prior to turning on this feature and was finally sending ~3 weeks later. So I deleted the "pre-dated" invoice and created a new one with the same invoice content and voila!, the pay invoice button showed up. Hopefully this helps anyone with this version of the problem. It seems this issue manifests itself in multiple ways. Good luck, all!