You do not need any certificate to teach as a community tutor. Perhaps, they decline you since they have an exceeding number of applicants. Try later, it happened to me once as well.

I don't believe there's any secret! I'm really not sure why your application wasn't accepted (you're technically more qualified than I was when I started haha since you're starting with previous experience!), though perhaps it is like Zoya said and they just have too many applicants at the moment. I would definitely keep trying. Good luck!

Tip 1: GEt to know the site and check etiquette. For example, on some sites, multiple posting is normal; On others not so much :)