"Next star after <n> hours"

[Azuraell]

Hi ! Sorry if this is not the correct place to ask.

Thank you very much for this app, it looks and feels very nice.
I just picked it up to learn Hiragana and after a few, it stops showing new characters.

Right now I am stuck at 19, and I know all of them. I want to learn more but the app won't show them, and instead everytime I answer, I get the message in the title. It says 2 or 4 hours generally. Do I have to wait to continue ? And if yes, why ?

Thanks :)