Box 2d Platform game problem

when my character stands at the very tip of a platform it is not jumping?

Code Below:


_body.m_userData.width = 20;
_body.m_userData.height = 50;

private function jump():void
{
if (_body.GetLinearVelocity().y > -1)
{

   var Hit = GetBodyAtPoint(_body.m_userData.x, _body.m_userData.y+ (_body.m_userData.height/2+2), true);//Under

   var Hit1 = GetBodyAtPoint(_body.m_userData.x-(_body.m_userData.width/2)+2, _body.m_userData.y+(_body.m_userData.height/2+2), true);//Down-Left

var Hit2 = GetBodyAtPoint(_body.m_userData.x+(_body.m_userData.width/2)-2, _body.m_userData.y+(_body.m_userData.height/2+2), true);//Down-Right

   if (Hit != null && Hit.m_userData != _body.m_userData )
   {
      _body.ApplyImpulse(new b2Vec2(0.0, -GameConstants.jumpVelocity), _body.GetWorldCenter());//Applys and impuls to the player. (Makes it jump)
   }
   else if (Hit1 != null && Hit1.m_userData != _body.m_userData && Hit2 != null && Hit2.m_userData != _body.m_userData)
   {
      _body.ApplyImpulse(new b2Vec2(0.0, -GameConstants.jumpVelocity), _body.GetWorldCenter());//Applys and impuls to the player. (Makes it jump)
   }
}
         _body.m_sweep.a = 0;
}


      public function GetBodyAtPoint(px:Number, py:Number, includeStatic:Boolean = false) : b2Body
   {
      // Make a small box.
      var px2 = px/Environment.RATIO;
      var py2 = py/Environment.RATIO;
      var PointVec:b2Vec2 = new b2Vec2();
      PointVec.Set(px2, py2);
      var aabb:b2AABB = new b2AABB();
      aabb.lowerBound.Set(px2 - 0.001, py2 - 0.001);
      aabb.upperBound.Set(px2 + 0.001, py2 + 0.001);

      // Query the world for overlapping shapes.
      var k_maxCount:int = 10;
      var shapes:Array = new Array();
      var count:int = Environment._world.Query(aabb, shapes, k_maxCount);
      var body:b2Body = null;

      // The loop that seeks for the body:
      for (var i:int = 0; i <count; ++i) {
         if (shapes.m_body.IsStatic() == false || includeStatic) {
            var tShape:b2Shape = shapes as b2Shape;
            var inside:Boolean = tShape.TestPoint(tShape.m_body.GetXForm(), PointVec);
         if (inside) {
               body = tShape.m_body;
               break;
            }
         }
       }
      return body;
   }