Hi, first of all, watch the video on the link below for my issue.

https://www.filemail.com/d/ugbekzuviaibvfa

As you see, my chain slips on the first cog. It also slips on the second, third and fourth cogs while pedaling.

My chainring has been replaced on last November and I rode around 2,000km with it.

My chain has been replaced last month and I rode around 300km with it.

My cassette is quite old. I rode around 9,500km with it. However, the first and second cogs only have been replaced last month.

My jockey wheels have been replaced and I rode around 300km with it.

My chain is properly lubed and cable tension is alright. No issue with derailleur nor bend on the derailleur hanger. Chain link is ok too.

I am still in lost what is causing the chain to be slipped.

Someone can help?

I am not going to click on that link, but there are many threads on this. When you replace a chain, you must typically replace the cassette as well. They wear together. If you don't replace the cassette, the new chain will skip around a bit.

It is a file upload site. I cannot attach video on the thread.

This is a classic case of a new chain and a worn cassette. The way to find out if a cassette is worn is if the chain skips when a new chain is installed. You need a new cassette, and don't ride this combination much because the worn cassette will result in a chain that will skip on a new cassette.

I hesitate to download a file and open it on my computer too. It's just not good practice. There are other sites that will allow you to upload videos and they can be played or streamed directly from that site. YouTube of course is the most popular. imgur.com is another place that will play a video without having to download it.

As to your issue, do you get the skip while just turning the crank by hand when your wheel is off the ground? If so, it's probably not worn cogs or chain. I'd think it'd be the adjustment is off a little. So a quarter turn of the barrel adjuster or inline adjuster might solve it. It might also be that your DR hanger is ever so slightly bent.

Usually you have to put some power into the pedals to get the chain to skip from worn gears or chain.

All the components did work together correctly before didn't they? If there was a slight issue after you changed something but ignored it and now it's gotten worse, then maybe what ever you changed then wasn't compatible with the other stuff.

If you are referring to the smallest cog there is probably a problem with the fit or lubrication of the chain. If the problem is with the largest cog I would remove it and place it on a flat surface to check whether it has become dished. A sticking derailleur can also cause problems and having the bike on a shop stand and turning the pedals and shifting through the gears is the best way to check this aspect of the drivetrain.

You should get 3-4 chains per cassette (without skip) and 3-4 cassettes per set of chainrings. 2,500 kms/1550 miles per chain is a rough guide, but use a chain checking tool. With Sunrace and Microshift cassettes you can get minor skip on chain #3 for a week or so before the chain settles down on the older cassette, while in my experience Shimano cassettes will often allow for four chains. SRAM I don't know about.

The lesson being, change your chain more frequently to maximize cassette and crankset lifespan. Some may change individual cogs on the cassette, but the interplay of new chain, new cogs and older, worn cogs can make for ugly shifting. It may seem like spending more money on consumables, but in the long run, it saves you money on new cassettes and chainrings/cranksets.