I've also been having this problem for a while. I have a lot of apps installed but not a single one of them can be moved to external storage - the "Move to SD card" button is always disabled on the app information screen. This doesn't seem normal to me.
I recently cleared all user data and started afresh, but still I can't move any of my apps to SD.
I'm using MIUI 2.10.12.
Any idea what is causing this? I'm starting to run out of space as many apps are quite large these days!