Rob Yeung's Answer:
Have you tried talking to your boss about how you feel? The only way to find out what is going on is to ask your boss – be direct, but polite, in raising the issue with your boss.
Ask your boss to set up a time that you can speak about your performance and what you could be doing to be more helpful. Pick a time when you know your boss won't be interrupted and will be in a good mood. Then prepare what you want to say.
Tell your boss that you've been feeling a bit sidelined lately, and ask if there's any specific reason for it. Try to talk in the first person (e.g. "I've been feeling sidelined" and "I get the impression that the new recruits aren't responding to me") rather than the second or third person ("you are sidelining me" or "the new recruits are not responding to me") because first person statements sound less accusatory.
Make it clear to your boss that you are merely expressing your opinions rather than making claims that you know the facts of what is going on. Your boss may not even realise that you feel the way you do.
Alternatively, it could be possible that your boss has changed how he or she is making decisions for good reason – possibly to do with your performance relative to those of the new arrivals. Then ask your boss to give you some feedback on what you've been doing well and not so well recently. Ask specific questions to find out what you could be doing better. Resist the temptation to become defensive and listen to what your boss says.
If your boss says that there is nothing to do with your work performance that needs changing, then reiterate how you have been feeling recently and see if you can work out with your boss a way of structuring the work within the team to help you feel more involved.