it is pretty common to receive general feedback after a consulting interview. The feedback is normally correct, but so far that there are no clear examples of what you should have done it is difficult to make it actionable.
In your case in particular it is not clear what the interviewer did not find convincing in your experiences. Also, it is unlikely that you did not pass the interview only because you got stuck in a calculation, with everything else ok, thus there are probably additional factors to improve.
I would say there are three options for you to understand what you can improve, not necessarily mutually exclusive:
- Post on the PrepLounge forum the specific questions that you think you could have answered differently and your answers – does not allow for live feedback, but it is free and with potential multiple sources
- Replicate the questions with experienced friends/Preploungers and ask them for detailed feedback – it is dependent on your contact knowledge, but it is free and with live feedback
- Replicate the questions with an expert and ask him/her for detailed feedback – not free, but allows to receive live feedback, a complete professional response and information on your additional improvements
Hope this helps,