MoviePlus5 mp4

We have recently installed MoviePlus5 in two classrooms on our school network. The codecs have recently been unlocked and appear as 'unlocked' in codec manager (when logged on as either a student or member of staff). However, students cannot import mp4 files - it states the correct codec is not present. When imported under a staff logon, all works well. The instructions regarding registry files has been followed and checked by Serif, but still no joy. I thought it could be a permissions problem and granted relevant permissions to the program folder, but still not working. Can anyone help?