Dropbox Mobi Export to Kindle App

[onthevirg]

I'm running into a strange problem where Dropbox no longer exports converted MOBI files (converted with Calibre) to the Kindle app on any of my devices. I did this a week or so ago and it performed as expected but when I tried to do it today it failed on two devices (Nexus 7 - 2012 & Samsung Tab A). Sometimes I know it takes a while but I exported the files this morning and they still aren't syncing up. I even tried removing the newer device to see if was related to that with no luck. Has something changed with either the Kindle/Dropbox apps so this functionality is no longer supported? If that's the case can someone recommend a EPUB/MOBI reader for Android? Thanks.