I've been having the exact same issue for some time now. I recently renewed my service and updated my program to the 2011 version and the problem is still there, only now it is worse. It used to be a simple matter to reenable the real time protection, just had to do a few extra clicks since the first option when you click on McAfee always shut back off seconds later. Now when I reenable it the same way it says it is on, but both the script checker and buffer overflow checker are off, and if I click home and go back to real time scan it says that it is still off. This is increadibly frustrating and it makes me think that real time scanning just isn't working at all....
:edit: I fixed the scan problem is fixed, I managed to carry out a normal full scan. :edit:
Combine that with the real time scanning being completly buggy and you have a very dis-satisfied customer. The fact that Mcafee has known about this real time scanning bug for over a year and done nothing is even more disturbing and if a fix is not released soon I will be getting a different anti-virus program and will never buy a product from mcafee again. A poor product and poor customer support, especially considering how importand anti-virus software is in this day and age.
Realtime scanning being turned off could be caused by many things and from my experience those could include unwise use of registry cleaners, competing software installed or even infection.
Try running an anti-malware product such as Malwarebytes Free, update it first and let it remove anything it finds.
If you have any software that duplicates McAfee's in any way please post that information.
Make sure that you operating system is totally up to date, especially your browser(s) and in the case of the latter make sure they are set to their default settings.
Lastly, your best bet in this instance is most likely Technical Support Online Chat linked under Useful Links above as they can link with your machine if necessary and check for problems that might be contributing to this situation.
At the weekend staff are scarce on this board.