You have to update your scroll bounds! That's the reason I think, if the camera doesn't render those tiles they don't collide
The camera size is for example 640x480 but if your tilemap is 1024x760 only the area cover from (0,0) to (640,480) wil be collidable. You have to set FlxG.camera.setBoundsRect(0,0,tilemap.width,tilemap.height, true), the boolean value advice to the camera to take into account the non render piece of tilemap (the area outside the current camera viewport).