The cause of this issue is either one of two things:
A) A loose, or intermittent coaxial connection from the wall to the box (there is an RF fitting at the wall jack and also one on the box itself).
B) Comcast needs to send a fresh signal to the box which essentially reboots it's software and can cure this glitch.
So if you're comfortable removing, checking and then reconnecting the RF (coaxial) fittings at both the wall output and also the back of the Comcast box - that is the first step you should try.
Make sure you do this with the Comcast box unplugged from wall power (remove the AC cord). Then once the connections are reconnected, plug the box back in and wait for it to reboot.
Once rebooted try to access On Demand again, and if you're still having the same problem, then you would need to contact Comcast and have them refresh the signal, or in the worst case scenario - replace their box (as it would indicate a fault in the box itself).
Let me know if you need further help. Also, please do click a 3-5 STAR rating for my efforts before you go. That is how I am credited for my efforts by the site.