Android PlayerPrefs not saving?

I seem to have a weird bug, which 'sometimes' on 'some devices' leads to PlayerPrefs not saving anything. has anyone else experienced this or has any idea how to fix that bug? The really bad thing about the bug is that although it seems to appear regularly (approx. 1% of our users) I haven't been able to reproduce it once with any of our devices, so Im stabbing in the dark here. For starters I would be interested in what circumstances would prevent the PlayerPrefs from saving anything? Would insufficient space would lead to data loss? Could it be a permission thing? I have no idea. One user though reported it went away after reintstalling, but that doesn't always seem to help. Also I had users reporting it stopped working after the last update, but then again it still works for the majority (99%) of the users.