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).