I use the Yahoo app for my phone and I was expecting it to be fully operational. But I have noticed lately that my email in the desktop is not synched with the same email in my phone meaning my updates in the desktop is not reflected at once in my phone. There are times that it takes days for my email to be synched. However, my updates on my phone reflects instantly on my desktop.
Maybe clear out your clean out your cache and cookies and go thought your email settings. In gmail it also does it ever hour unless I click on the little death rate to say search