Why does some time only one speaker work?

The fact that the problem arises only with a particular song, or video clearly indicates that the issue is with your song/video. Had it been an issue with the speakers or the device, you would have faced the same issue with all of your collection.

The issue can be fixed by using a video editing software. You may even try it out with Audacity or such other software. You will need to extract the audio files from the video under question. Once you extract the sound, it has to be synced with the video again. You may need to check some tutorials to do this.

This doesn't have to be an issue of the equipment. This sounds like audio rendering issue. Sometime audio editors can be configured to render single stereo sound. This is applied in case of music that is meant to be sync with the speech. So often you may find that such tracks make use of single speaker. However it is rare that the song itself switches to single speaker. In such case that could be the bug during the rendering process.

Often effects such as reverb and synths are configured which often may not render properly and lead to such issues. I had similar issues when I used audio editors such as ACID.

You can check if the audio rendering has this issue. And most of the audio editors such Sony ACID and GoldWave let you fix this. And you can make the sound to be heard from both the stereo. In order to do this you have to adjust the sound file accordingly. Problem such as this can be fixed if you know how to edit the sound in audio editor.

I have experienced the same, and just to solve the problem, I found out that:

1. Some songs when copied from the original into another format could be by mistake recorded on MONO while it should have been recorded in the STEREO mode.

2. Some times some particular site from where we download the song don't record the song in the required quality. The sound quality deteriorates and so this results into this single output of the song.

3. At times the headphones too create some problem. some sort of loose connection to the music system too results into this problem.

So, what you need to do is:

a) If you are downloading a song, try to listen to it first, and if it sounds okay, then only proceed.

b) always download the song in highest sound quality.

c) Adjust your headphone while listening, sometimes a half insertion of the jack might work well than inserting it fully.