The most common issue is this guy falling off or not in the right spot yielding the device not controllable. If you have an RF or IP controllable system, meaning you can operate your system without pointing directly at the components, IR emitters are, in fact, part of the control system.
Cable boxes, media / bluray players, game systems all have different places where the component receives IR. Best practice to reaffix the emitter is this: in one hand, hold the universal remote and press a button that you know has worked. Then slowly move the IR emitter over the front of the component until the device reacts. Use a hot glue gun or a piece of tape to reaffix the emitter to its appropriate place.