Re: Aikido against strong people...
That usually means that you do not have enough strength to make the technique work.
Now strength, in this case, is not muscular strength. Rather, it is the strength that arises from your body working to effect the technique. And while the technique makes such work efficient, having your body's musculature trained to maximize the effect of the technique will lead to the technique working on the stronger opponent.
To improve, perform your kotegaeshi on a someone who can resist it, and feel which muscles of your body work when you execute the kotegaeshi. Once you identify the muscles, work to remove the tension from the muscles whenever the technique is done, and once you can relax the muscles while executing kotegaeshi, the technique should be able to work.