Folder syncronisation

If I start the client, the sync is working without problems, but the incoming folder will not synchronized.
As recently as I choose another folder and come back to the incoming folder he’s refreshing and I can see new mails. Is this normal?

No, this is not normal.  Is this an IMAP account?  Who is your email provider?

Yes this is an IMAP account. Thunderbirth and Outlook don’t have this problem