Biggest general tip: practice and experience. Do some mock interviews. Sit in with some other interviewers just as an observer. Be
extremely critical of your own performance as an interviewer.
It feels silly. However, it helps a lot. Sitting in on an interview is especially useful. You can pay attention to the reactions of the interviewee a lot better when you're not running the interview. You will recognize lots of cringeworthy moments--pay attention to them and try not to do them yourself. Asking other people in your company to answer your interview questions will pinpoint places that are vague or unclear and possibly even incorrect. It also helps calibrate--if already employed people can't pass your interview loop, your interview loop sucks.
However, interviewing is a human thing, so like any social question, there are no hard and fast answers.
Three particular pieces of advice I give:
1) Basic courtesy and professionalism
Good grief. Why are people so terrible about this?
Don't leave an interviewee alone. Be on time. Remember their name. Try to call them what they want to be called (my name has various shortenings that I don't care about--some people care very much, though). Offer them drinks and bathroom breaks. Read their resume more that 30 seconds before entering the room. Actually read their resume. Don't make them drive for lunch. Have their reimbursement check by the end of the interview. Thank them for their time.
This industry seems to have so few people willing to provide even basic courtesy and respect that it makes me cringe. This industry is small and the wheel turns a lot--that person you are interviewing may be your interviewer in 5 years and absolutely will remember you if you are kind and professional.
2) Listen. Really listen. Shut up and listen. Pay attention to the person in front of you and try to make them feel like they are the most important, most interesting person in the world.
The person IS important. You're trying to hire them and make them your colleague. Treat them like it.
When you do this, people will tell you a lot. They almost can't help it. You're not trying to trick anybody; you just want them to be who they are professionally.
3) Start from slightly above neutral and give minus and plus assessments.
Sure, if they can't pass your FizzBuzz equivalent, you don't want to hire them.
However, too many interviewers start from default "You're flunking" and it's very difficult to get out of that. This is a junior person--by definition, they're going to have a difficult time moving your needle in the positive direction.
Given this is a junior person, the interviewer needs to work to hunt for positives. Your questions need to have enough open-endedness that an interviewee can impress you to varying degrees. If you find something that you deem positive on their resume, ask them about it and give them the opportunity to score some points.
Finally, one thing to do is simply be honest about your job positions. Most positions are not rocket science--they are standard coding positions with some business specific quirks. If your junior candidate got a degree with a better than 3.0 GPA from any decent (read: something like ABET accredited) program, they have pretty much already demonstrated that they have the motivation and acumen to do your job.
So, if they "didn't pass your loop" you should have a really clear reason why you are not hiring them. If multiple junior people aren't passing your interview loop for junior people, the problem is more likely with the interview loop or your job expectations than the junior candidates.