The memory scanner occasionally may “think” it succeeded although it actually just found some bogus addresses that don’t do anything. Try again from a different spot, also a game restart, and even a PC reboot (not turning off/on, which doesn’t do a full reboot on modern PCs) may help sometimes to make sure the RAM is uncluttered.
If it doesn’t work at all, you’ll have to play without the memory scanner. You can turn off applying the scanner result in the vorpX menu after several unsuccessful attempts if you are stuck with non-working head tracking.
Unfortunately there always is a certain chance of failure, hence the info message that is shown during each scan, in some games more than in others. Automatically trying to figure out a single memory address that contains the camera rotation is a fairly complex matter.