Skipping bolts on seatpost , please help?

Question

Couple weeks back I bought this aluminium single-bolt seatpost .
Now, just 20 days later, the nut that tightens the the mechanism to fasten the saddle rails to the seatpost is skipping around and not tightening.
Why ?
I was testing new saddles on this seatpost, so tightening/loosening. Then, after I got done testing, I tried to put in the chosen saddle.
When I tried to tighten and fasten it, instead of the bolt moving into the rectangular nut, it got stuck and the nut started to move along with the screw, causing the entire upper metal plate to spin and get stuck as I tightened. This had happened a couple times before too. So, I tried holding the metal plate in place with my fingers while tightening this bolt to keep the upper plate straight. Impossibly hard, and I wasn't able to stop the metal plate from turning.
So, I presumed lightly oiling the screw system should help, thinking it has seized. I went ahead and put a couple of drops of oil in there. Now beyond  certain (very loose) extent, the nut and bolt would just skip and slide and rotate around but not tighten anymore. Frustrated, I though that the oiling was a mistake, so I removed the plates, bolt and nut assembly and degreased it thoroughly and properly with a brush and degreaser.
I reassembled the plates, bolt and nut but it's still skipping and not tightening.
I seriously doubt that the bolt is worn out from such light use.
What is the solution ?  I'm so clueless and haven't been able to ride at all since days now...
EDIT :
Link to close-up, labelled pictures of the parts of the seatpost for your understanding of their condition and quality : Here.
I confirm through careful observation that the bolt's thread seems damaged, and , from the observation of others here , also that of the nut's.
This damage causes the skipping and sliding, and I will be going to a hardware store and getting a new nut and bolt or just one depending on which is needed.
I am not sure but AFAIK both the bolt and nut appear to be aluminium.
Over-tightening might have been the likely issue , since at sensibly tight levels, the clamps failed to hold the saddle which would slide noticeably under climbs . Hence, I had regularly over-tightened it (like a true Jeremy Clarkson gorilla-with-a-spanner) and these cheap screws & bolts just couldn't take it.

Criggie · Accepted Answer

₹ 555 (Indian Rupees) is about $11 NZD or about $7.50 US, 5.64 UKP or 6.27 Euros.
So you've got a seatpost that is incredibly cheap and likely made of low-grade metals.  Zooming in a bit shows the terrible surface finish, and the inside won't be any better.

By repeatedly alternating between tight and loose, the threads inside have become "stripped" and there's not enough metal left to hold the bolt.    I bet you found little half-moons of metal flaking out of the hole when removing the bolt fully.
There's a chance you over-torqued the bolt too, which uses the bolt's thread as a kind of lever to force the metal threads off the sides of the hole.
You may be able to put a wider bolt in after re-cutting a new thread with a tap, but given the cheese-nature of the metal, it won't last very long.  You can also fit a replacement thread, but the tools for that cost more than a decent seatpost.
On the other hand, if its the bolt which is missing its threads, and the threads inside the clamp are still okay, a replacement bolt would resolve the problem for now.  Do make sure its a quality bolt.
Buying a better-quality seatpost will fix your problem.  For me, a low-price good one is four to five times as much.   You don't need to spend excessively, but based on the pricing and what has happened, it was a piece of junk.   You might also explore returning the item, though that is up to you.

Your second photo clearly shows no threads left in the square "nut"  If you could replace that with a thicker nut it would hold for longer, but some other part could fail too.  I would expect the pressed metal rail holder to give up next, or maybe the bolt itself.

Skipping bolts on seatpost , please help?

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