Interview or assessment feedback

You can ask for feedback after the interview or assessment: Feedback helps you to learn what did or did not work well and to make adjustments for your next interview

Feedback will be most helpful if:

You genuinely wanted this particular job
You feel you gave a good interview
You got through to the later stages (if it was a multiple-stage process)

The organisation does not have to give you feedback and might not be willing to for a number of reasons:

Legal risk – they might say something that leads you to think that you are being discriminated against. Many companies will say as little as possible to avoid this risk.
There is no feedback to give – When there was nothing you could improve on. Another candidate was just a little bit better. This is good news, and although disappointing that you didn’t get the role, you can take more confidence into your next interview.
It’s uncomfortable for the interviewer – no-one likes giving bad news, having to explain it is even more awkward and they may worry that you will get upset or angry.
It takes time – when they may be under pressure. Helping you is not going to be their top priority.
They are only qualified to say why you did not get that job – they are not qualified to say what may work for a different organisation.

The feedback they give you may not always be helpful, as unfortunately they may not tell you the real reason you didn’t get the role