Congratulations on making it through to Round 2! You should take a lot of comfort from this - it means that you've met the ‘bar’ for the interview and it's now a question of whether you can ‘out score’ the other candidates to receive the offer.

Interviewers will always try to give you feedback on where you can improve - so the feedback doesn't mean that your structure was necessarily bad. What it sounds like happened (and what is really common) is that you tried to fit a generic structure to the case. Consulting Firms know that people can learn multiple structures and so are coming up with increasingly inventive cases which will defy any of the structures that you've learnt. 

A common mistake that candidates make is taking a non-standard question (e.g., if you drive a taxi are you better off driving around the town hoping to be flagged down or going to the airport to wait to pick up a passenger?) and trying to use a standard structure to answer it. In this example you'd probably think about fixed costs, variable costs and revenue - but you might not think about other things such as the safety of the driver (what time of day is it? If it's night time are you more prone to attack if you're in town?) etc., 

My advice is to practice non-traditional cases and stepping back to think about yes how the frameworks can help to inform you answer, but what else do you need to consider to fully answer the question. You can then merge the two to essentially either tailor the framework to the question or, if the question is completely non-traditional, create your own structure on the spot. 

Good luck!