Why are users unable to verify even after clicking on the activation link?

Hey, we bought this plugin a month or two ago. It’s been working pretty well. However, ~1% of users seem to be having issues verifying their emails and we get a couple of complaints every day that they click the verification link multiple times but are not able to log in. It seems to mostly affect Yahoo and Hotmail users. Any suggestions for how to resolve this? It’s not a great look when we force verification to login but they cant get verified! Thanks!