Unable to advance from Nucleotide Mixer to Gene Synthesizer on iPod Touch

My niece is trying play your app on her iPod Touch running software version 12.5.1, and has completed all puzzles in Nucleotide Mixer – multiple times over now actually – but the app isn’t letting her advance to Gene Synthesizer. Is this a known issue? She isn’t logged in, but neither is her sister (who isn’t logged in either but is using an iPhone and has had no such trouble).