There are two things that are possible to be causing your concern. Over time the brake switch and or the cruise switch itself can go bad. If the brake switch is worn out it will stick on this will not allow you to resume or set until it unsticks. This can happen after you push it down a few more times. The one way to know if this is happening the brake lamps would still be on when you are unable to set or resume.
If this is not the case I suspect a bad switch for the cruise. The contacts can be worn out so when you attempt to set the cruise it is not getting the signal from the switch.