Users are not automatically logged in after they purchase a membership

I am embarrassed to ask this, but I am not seeing my users automatically logged in after they have purchased a membership. Their membership is accepted and I see it in the Admin Backend and they can log in manually. Furthermore, I have been testing my site extensively for the past few days and I always saw my test users automatically logged in after they purchase a membership. Now all of the sudden that has stopped. Am I missing a setting or something?