If you used true key limes but the pie is too sweet & doesn't have a strong lime flavor it may be about timing. The limes may not be at their peak season. They may also be too old or weren't ripe enough. One way to tell if they limes were not at their peak is if it took 6 or more regular sized limes to yield enough juice; less juice means less than optimal fruit. If you used regular limes instead of key limes, the flavor will not be as intense. This is why in my substitution section I recommend using a 1:1 ratio of both lemon & lime juices.