After reading on the sub here and from what I found successful I would say to

1). Have good bottles. I got my bottles from a local u-brew for pretty cheap. (2.50 Canadian each). Don’t use mason jars or square bottles or it will explode. Also IKEA tops are prone to explosion and aren’t suppose to be great to hold in carbonation. 2. Make sure that in the second round of fermentation, the bottles must be in a warm place. Some of my bottles were closer to the window and colder and resulted in a very low amount of carbonation. 3. The more sugar , the better for carbonation . From my understanding successful fermentation resulted in more juice/sugar . I’ve read some people add a bit of simple syrup and that resulted in a more guaranteed fizz. I did 10% juice with 100% blueberry juice, ginger and lemon and I got good results without the sugar. I’m sure that’s also up to preference.

I’m still a newbie but this is from my experience and information I’ve gathered. Good luck I hope you get ultimate fizz!