In your profile tab in the top right, click settings in the drop down menu. Once you do that you get to a list of settings for your FollowUp.cc account. The first box affects all things reminders.
That first check box (if checked) will cancel any reminder when someone replies to your email with the reminder attached to it. This is recommended as a best practice to not flood your inbox with emails you already know have been responded to.
We also reccomend keeping the rest of the boxes checked with the exception of the last one unless you're using FollowUp.cc with a team.