K-9 doesn't download new emails although there are new an unread emails on my account!

I’m using K-9 Mail on my Android and i’ve tried to download my email with K-9: no new emails. It was a bit of time i didn’t receive any email and the thing sounded strange to me. So I checked my email account via browser and i’ve found a lot of new and unread emails that K-9 didn’t download.

How can i solve this?

Thank you all,