First of all be sure to have a good internet connection, can try wifi also if possible.
Start your app and go to please go to settings/diagnostics/reset blockchain.
Let us know if that helped, if not we will do more advanced step.
I am having this issue as well, but on the Windows client. I cannot find the reset blockchain setting. Under tools there is a wallet repair tab
Andrew have you solved your issue already?
Pegah Nikzat