Board Thread:Love Live! School Idol Festival Lounge/@comment-28259688-20160625100611/@comment-26161729-20160625125921

it can be either a phone problem or most likely, a problem with the game--

if it's a phone problem, restarting the app and your phone might do the trick-- clearing up your RAM or making sure no other apps were open might fix it too--

but if it's a game problem, then your game files are probably incomplete or corrupted, and this probably occurred during the download-- you could either wait for the next update (downloading new patch) or reinstall your game- if you decide to reinstall, make sure you take note of your transfer code-- :D

Reference: SIF Reddit Link