Collisions - have some questions

Hi
I have some questions regarding collisions.
In my app I’m making groupds of objects, one of these objects is seonsor which is responsible for collision detection. here is the example:

 local wallObj = display.newGroup()  
  
 local picWall = externParams.uiutils.getThemeItemName( theme\_id, wall\_id )  
 wallObj.wall = display.newImage( picWall, true )  
  
 wallObj.sensor = display.newImage( "sensor.png" )  
 --sensor.alpha = 0  
 wallObj.sensor.type = wall\_id  
 wallObj.sensor.obj = wallObj  
 wallObj.sensor.y = wallObj.wall.y + wallObj.wall.height / 2 - wallObj.sensor.height / 2  
  
 wallObj:insert( wallObj.wall )  
 wallObj:insert( wallObj.sensor )  
  
 wallObj:setReferencePoint( display.CenterReferencePoint )  
  
 physics.addBody( wallObj.sensor, "static" )  
  

I move all this objects manually by setting their x and y values. My problem is that I don’t recieve collision events when one sensor overlaps another sensor. Is there any way how I can fix this?
Thanx! [import]uid: 67485 topic_id: 14478 reply_id: 314478[/import]