After 3 tries, if your move doesn't work, at least you have studied your opponent's respond well. You can then take advantage on his respond.
For example,
1. You kick your opponent's groin. He drops arm to block your kick.
2. You kick your opponent's groin again. He drops arm to block your kick again.
3. You kick your opponent's groin the 3rd time. He drops arm to block your kick the 3rd time.
During your 4th groin kick, when your opponent drops his arm to block your kick, you punch on his face. Of course you can use groin kick, face punch combo on your 1st try. But if your groin kick can knock your opponent down, why do you even bother to punch? Your opponent may be fast enough to block your 1st groin kick. But he may not be fast enough to block your 2nd or 3rd groin kick. When you deliver your 4th groin kick, you may just use only 30% power and speed in your kick.
When you attack, your opponent can
1. escape of your attack.
2. counter your attack.
If your opponent counters your attack, of course you can take advantage on his counter. But if your opponent just escapes your attack, you can only take advantage on his backward footwork and weight shifting. If his backward footwork is faster than your advantage footwork, you can't reach him.
Take single leg for example. If you shoot in 3 times and your opponent is fast enough to move back for all 3 attacks, when you play defense, the moment that your opponent's steps in, you shoot at his leading leg. In other words, when you play defense, you let your opponent to close the distance for you.
A: What can I do if every time I attack, my opponent always moves back?
B: You let him attacks you instead.
A: What can I do if my opponent won't let me grab on him?
B: You let him grabs on you instead.