scripts for collision not working

I'm having a problem with my game. I have a forklift truck and attached to it is a gameObject. I wrote a script that when the forklift collides with a fire particle, the gameObject is deactivated and points are added. Here's the code:

  function OnParticleCollision(touch : GameObject){

  if(touch.gameObject.name == 'Flames' && copperCubes.active ==true){

  Points.myPoints += 10;

  }

Now the code works fine until I restrict the forklifts rotation on the x and z axis. If I restrict it's rotation there doesn't seem to be any collision being detected. How do I get around this?