Mail forgets yahoo password periodically

I'm running an iPad mini 4 with IOS 9.1

every once in a while, when I open the mail app, I get the message that it doesn't have the correct password for my yahoo account. It does this rarely on my iPhone 6, which is also running the latest IOS. This issue has been around for a few IOS versions now. Is there a known fix? I only access the yahoo mail from my phone and the iPad. I don't even use my PC for it anymore. When I re-type the password, it accepts it and gets the mail. Anyone know why this happens, and when a fix will occur? Thanks for your responses!