Re: when can I call again?
Call again. They might still be interested. Sometimes in organizations it can take an incredibly long time for a decision to be made.
Even if they've decided not to hire you, they may look at you again if you continue to show interest.
If you do get to talk with someone and they are definitely not going to hire you, then it doesn't hurt to ask why. Just be sure you come across as being willing to learn and improve.
If you don't get a particular job, it's not always something you did or didn't do. It may be that someone else with more experience in that particular area applied also and they will be able to work independently much more quickly than you.
Unfortunately it's not uncommon to never hear a word back, not even a "Thanks but no thanks." Applicants are expected to send Thank-You letters but employers very often don't extend the same courtesy.
You don't want to be a pest, but I think if you haven't heard back in a week after an interview, it's OK to call and then call every week thereafter until you hear something.