Is it possible that your smash is too flat? If you're trying to limit the possible angles of reply by playing to the centre, it requires that the opponent is forced to hit the shuttle from well below net height. Steepness is more important than power here, as a flat smash away from the body is actually one of the easiest to counter attack – they can play a net-shot, push, drive or clear (a steep smash limits them to either a net-shot which would be risky with your partner covering the net, or a clear which just invites another smash). Also, any time you allow your opponents to hit the shuttle from around net height (or higher), your partner should not be at the net, or she'll be left totally stranded (however quick she is) – you really need to switch to playing sides at that point.

You say “my opponents do not need to stand deep at the back court”. This exactly the situation where you should be smashing into the body (ideally the racket-side hip) just to keep them honest. If your opponents are waiting further forward they can take the shuttle very early, which allows them to play better, more attacking shots. However, it also means that they have less time to react – especially to fast shots hit straight at them. A few good body smashes should force them back away from the net, and that will open up the front of the court, making your drop shots (and steeper smashes to the centre) far more effective.

Just another thought, are you trying to smash too often or from the wrong position?

What I mean by this is you may be too far back when you are trying to smash. If your opponents put a good length clear to you then you won't be able to smash to the floor, no matter how good you are.

If my opponents play a good clear then I will try a drop to the centre of the net or simply put a high deep clear back to them. If my opponents creep forward to attack the drop shot or put pressure on a potential smash then I may try a flat clear to rush them backwards and force a weak cleared return.

Drop shots are often the best way to force lifts. If you end up in the mid court with the shuttle at or just below net height, push the shuttle to a gap at the net and follow aggressively in behind it. A lot of players will lift on seeing someone bearing down on them at the net.

Being patient and working the rally to give a smash opportunity is key.