I have this "old" Lenovo A2107 tablet which I rooted years ago (using a download from http://androidforums.com/threads/a2107-root-mods-and-rom-discussion.661261/, tho running its root.bat script by hand from my GNU/Linux machine) and that worked fine.
Then I passed this tablet to my daughter for a couple years, and now I got it back.
/system/bin/su is still present, still the same date and size, still mode "rwsr-sr-x", still the same firmware (Android 4.0.3) so all looks fine, but when I try to run it from a shell, it just tells me "Permission denied".
Any idea what might be going on? What I could try to do to track it down?
OK, for those curious to know, I figured out what it was: the /system/bin/su program either accepts the request as-is (depending on who asks) or passes the request to some other program (Superuser.apk).
As it turns out, I still have /system/bin/su but Superuser.apk was missing. Luckily, /system/bin/su worked fine from the "adb shell", even though it said "Permission denied" when used from ConnectBot and other terminal emulators. So I used "adb shell" to get root access and manually copied Superuser.apk into /system/apps.