Why does meditation not succeed?

The following is an excerpt from a samvaad (dialogue) session with Acharya Prashant.

Questioner (Q): Acharya Ji, while meditating I feel silence and joy for some time, but after that, I’m not able to continue it and start feeling uneasy and want to come out of the meditation soon. Why does this happen, and does it mean that I’m not reaching the meditative state?