Why do we sometimes have an itch that isn't relieved by scratching?

Sometimes an itch is caused by an irritant on the skin, which causes nerves to send a signal to the brain that there is something wrong at that location. Scratching the itch sometimes overwhelms the nerve endings with signals and causes the itch to go away. (This is what usually happens when we feel itchy).
Sometimes, however, the itch happens from the other end - there is a misfire or false signal in the brain at the location where a nerve signal would be received, and we interpret that as a signal coming from the nerve, where there is actually nothing at the other end to cause a signal to be sent. However, scratching the location where we perceive the itch to be coming from can also overwhelm the nerve endings with signals, which can send signals to the brain and overwhelm the receiving end and cause the itch to go away.
Source: had a neurologist (¿ I think ?) girlfriend who did a study on phantom pain in amputees. They could stimulate bits of the brain and “trick” people into feeling their phantom pain somewhere else where it could be treated, or feeling itchy, sometimes on existing limbs; sometimes the itch would be on the non-existent amputated limb.
- Thank you!

Semi-related, I listened to a podcast the other day that discussed sokushinbutsu where some Buddist monks would try to self-mummify themselves as a ritual to attain enlightenment. One of the steps involved Urushi tea which comes from the bark of the Urushi tree, or also known as the Japanese Varnish Tree, and the sap from this tree contains toxic and abrasive chemicals that can cause a rash similar to poison ivy. They would literally be itching on the inside and had to use mind over matter to ignore the extreme discomfort.
- What a terrible journey to enlightenment. I'll take psychedelics, thanks.
- Did any succeed? Or did they just die and decompose like everyone else?
  
  A few did. They are then enshrined in the temple and ~~worshiped~~ (revered might be a better term?) as a living Buddha.

There are things besides surface objects that cause itchiness. Scratching only stops an itch if there was a surface object that was causing it.
For me personally, putting moisturizer on the spot gets rid of those itches most of the time

I get this sometimes, I think it's because the nerves that are actually triggering the itch are someplace else, kind of like when you get that itch inside your ear that you can somehow relieve by scratching the roof of your mouth with your tongue. I feel itchy now!

whenever i get an itch that i can't scratch, usually it's just because the itch is actually someplace else, so i gotta search for it.

I would recommend a food journal, odds are that you have a mild allergy to something like mustard or sesame with a 24 hour delay.

I get crazy bad itches as a side effect of my lupus medication. It gets worse with sun exposure.
Slathering absurd amounts of cooling lotion on the itchy spots overloads my senses enough to make me forget the itchiness until it subsides.

Hah. If I wanted to ask a really interesting question I'd ask why would scratching relieve an itch in the first place.

Because it's too deep in the skin. It's also why foot itches are so bad, because it has more skin so you can walk long distances.
- Except for the sensitive part where you can't scratch too hard. 🤤😵‍💫🤤

It has been my experience that this indicates that the itch is actually somewhere else. When this happens to me, I just start itching all over the place until I find it.
- Like the inner ear itch that gets scratched on the roof of the mouth. That makes perfect sense to me. Cheers.
  
  i think thats allergies, i was shocked to find out some people didnt know what i was talking about.