I have had the same issue with Tapatalk on my Iphone the only thing that seemed to solve the issue for a while was to uninstall the app and re-installing it, it would work for a while before doing it again.
Now that I switched phone and have an android I have not had that issue which lead me to believe that their is an issue with that app version for Iphone, and I only had issues since updating to a newer version, never had issue with the older version.