It initially took 2 weeks to schedule a meeting with the talent person, but they were then quick to follow up with the problem solving interview. The interviewer acted more as an observer rather than a peer and he was not particularly confident about Java as a language.

I solved the problem, but was unable to test the solution in the given time because of a misplaced parenthesis on Java comparators. I didn't spot the mistake by the end and neither did the interviewer. I think this is the reason for the rejection, but I can't be 100% sure since they didn't provide any feedback.

You should implement an agent scheduling solution.