This needs some explanation, here it goes.
To prove an adultery case there needs to be FOUR witnesses who saw penetration occur.
All their stories have to exactly the same, if one is different then the case collapses.
The witnesses need to upright, religiously and in other aspects. For example we would not accept the testimony of a person who is a known drunk or does not pray. Or a person who is known to lie.
As you can see there is a lot that is considered before a penalty can be applied.
One of the most conditions is that there need to be a Muslim caliph in power, or his deputy if neither are present then the penalty cannot be applied. There has been no caliph since 1924.
The Ottoman Muslim caliphate recorded everything and there was only 5 cases of this happening in a period of 500 years.
It is notoriously hard to prove. I hope this answers your question, please let me know if i haven't.