Invitation code feature not working 100%

Hello,

I have membership 2 pro installed on my site specifically for the invitation code features but it doesn't work 100% of the time. Today (7/4/16) I had someone else sign up without having to use one of the codes. I tried it myself and it didn't ask me for it. After deleting and reinstalling the plug ins (wpmu dash and membership 2 pro) it was working again. Can someone help me fix this issue so that I don't have to keep uninstalling and reinstalling? Thanks.