First, make sure your software is up-to-date.
Is the storage close to full?
Have you tried closing all of your other apps (home button double tap, then swipe up on all apps).
These can help rule out a few easy "gotchas".
If that doesn't work:
Try restarting the phone and perhaps even deleting and reinstalling the app (I hate it when techs tell me to do this, but it does sometimes work 😁).
Let us know how that works out and we'll go from there. Thanks for your patience–we'll get this sorted out!