Yahoo mail acts oddly on my iMac but works perfectly on my iPhone and Chromebook

Yahoo mail acts oddly on my iMac: new folders don't organize themselves, some emails never show up unless you "search" for them, emails don't show in the folder they were moved to, etc. "But", everything works perfectly on my iPhone and my Chromebook. So, there's a folder that only shows one email in it on my iMac, but on my iPhone it correctly shows six emails in it. Please help!