Without understanding a lot more in the way of details this is not something that will get a clear answer. But to understand some generalities:

- Every interviewer gets a vote and it is basically a straight up HIRE or NO HIRE. Then there is supporting commentary. If any one interviewer says NO HIRE then it kicks off fairly intensive discussions to determine why the NO HIRE vote. The hiring manager or, in some cases a person known as the "As Appropriate", then makes the hiring call. If the NO HIRE came from the "As Appropriate" then that is usually the final word. So you only need to have one out of several interviews go bad to have your chances go from good to poor.

- Microsoft is looking for long term hires, and thus you'll be evaluated against criteria that go beyond the specifics of a job. These used to be embodied in something called the Microsoft Success Factors. Do you show a passion for technology? Are you a long-term thinker? Can you get things done? Are you customer focused? Etc. These days some of the criteria are no doubt different, for example there is more emphasis on cross-organizational cooperation, but you get the general idea. You could be brilliant and quite technically competent but not display characteristics which indicate you would succeed at Microsoft. Amazon has their own set of principles, and if it doesn't look like you'd measure up to those then they aren't going to hire you either.

Let me give you an example here. One of my friends, a fairly legendary developer, actually got all HIRE recommendations, but when we sat down to discuss it we decided that he was a lone wolf who could never be part of a bigger team. You had to be able to give him a project and let him run with it with no interference, and we just didn't have a project like that. So we didn't hire him. He's had a couple of successful startups since then, because of course that environment suits him. Numerous former co-workers of mine at Digital were not hired by Microsoft even though many of us knew them and knew how good they were. But they either didn't measure up on some criteria important to Microsoft, or they didn't interview that well.

- A really tough one is that Microsoft will often interview for a specific role, and decide you aren't right for that role. For example, they may have been interviewing for SDEs (developers) and decided you were more of a PM (Program Manager). Or they might decide you just wouldn't be good at creating Xbox games even though you might be a fine developer elsewhere at Microsoft. But perhaps the team(s) where you'd be a better fit don't have any campus openings. Or they had already filled them. Under the best conditions you get called in for more interviews, or re-routed in the middle of the interview process. Under the worst conditions you have to re-apply.

- It is a competition, or at least a game of musical chairs. They had 20 people apply for 5 openings and they made offers to what they thought were the best 5. You might have been the 6th person.

Campus interviews are amongst the hardest in my opinion because you have so little to go on. That's why puzzles, white board coding, and other techniques are sometimes used. I can't really ask you about what you've done, so I try to see how smart you are and if you can think like a Developer/Test Engineer/Program Manager/Product Manager/etc. Now personally I am not a fan of random puzzles, but I will ask tricky questions based on what you took in school or what project work you've done. If you took a database course I will give you a transaction schedule and ask you what the right outcome is. I'll then watch how you work it through. I have hired people who get the answer wrong (particularly because transaction schedules (i.e., isolation) doesn't get much coverage in an introductory database class) but I could see their brain working well. I have had a few people nail the question and I've all but hired them on the spot.

Twenty years ago one of my friends expressed a good criteria for hiring. "Would you chase the candidate out to the parking lot trying to convince them to come work here?" Whenever we'd be debating on hiring a candidate that is the question he would ask. If the answer wasn't YES then we wouldn't hire them. So the challenge for any candidate interviewing for a job at any company is simple: make them want to almost stalk you until you agree to work for them. For a college hire that means they need to see you not just as someone who can get the near-term job done, but as a future super-star.

Ok, so you didn't get hired right out of college. What should you do? Find a good job elsewhere and work your ass off to show you have super-star potential. Then apply back to Microsoft and Amazon in a year or two. If you know someone at Microsoft (or Amazon) see if they'd be willing to be your mentor. If you don't know someone, look for an alumni of your University who is working for Microsoft/Amazon and see if they will do it. The mentor can help you with your career in genera and, assuming they have the Microsoft/Amazon connection, with preparing yourself for a better shot the next time you apply to one of those companies.