Well firstly, if you just asked a random shop girl, she may either not know or not care that there are vacancies. When I was at school, we went round town giving out CVs and one of the managers we spoke to (most were just normal staff) said it was good we'd seen him, because the staff usually just put the CVs in the bin. Lovely and helpful, of course!
Also, you're getting interviews. This is GREAT. Are you in sixth form or uni? If so, then you may find there's some kind of careers service that you could go to for a mock interview, this would hopefully get you some feedback about how you're performing and what you could do to improve.
If you get in touch with the people who conducted each of your interviews, you always have the option of asking why you were turned down, and what you could do better in the future. My partner had two unsuccessful interviews before he got his current job, and both were happy to provide this feedback. Chances are it might just be luck. If there are a few people applying for each position, and all perform well at interview, it might be literally just random bad luck that you didn't get it.