I have checked my own iOS app on my iPhone and I can see that it is updating just fine and working properly.
I highly suggest that you close all your app on the multi-task feature of the iPhone and try to respring the iPhone. This might be caused by several app opened on background.
If none of it works, I highly suggest that you check if it is the updated version of JotForm iOS app.
Try to re-install the app and login your account again.
Those troubleshoot suggestion should make the app work smoothly as I have no problem with my JotForm App on my iPhone.
Kindly let us know if you have further questions or inquiry.
Thank you. :)