Stucked wheel disks can be a problem the day you have to change a wheel or if you just wish to have a full access to the inflating valve. The Tomy bar method as described in the user's manual is the only one that applys for non vented disks, but for others I've found it easyer to do it by hand.
0-Vented wheel disks
All the wheel disks do not have vents. If they do, you can use your hands to remove them.
1-Suitable position to remove the disk
Palms up resting against the tyre, grip with your fingers. Pull firmly to unclip the disk.
2-Wheel disk removed
3-Give it a clean
Overkill unless you want to check its condition
4-Gripping washer that secures the centre of the disk
There are 4 to 6 of such washers on each disk. One or two may be missing with time. The center hole is 5mm.
5-Exterior cleaning of the disk
6-Before fitting the disk back in place
Applying a thin coat of waterproof grease (silicon) on the metal circular clip should ease further removal.