Pilot light goes off ?

We have Vailant gas boiler type vp5-ze 15. The pilot light has developed a fault. No longer it stay on. Usually when the pilot light goes off, you have to push and hold “Light on Switch” and push ignition switch at the same time to light the pilot pilot. This puts the light on. But you have to keep pressing “Light on Switch” for at least one minute. After that you let “Light on switch” to go loose. After this immediately the boiler fire goes on and the pilot light stays on. But now when you let the “Light on switch” to go loose, the pilot light goes off. Some times I have kept the “Light on Switch” pressed for as long as 5 minutes but it still gone off. Please give any advice you can.

For information:
“Light on switch”, which has 'light symbol' is an internal pressure switch fixed beside “light off Switch”. “Light off switch is used to put off pilot light.