I agree with the above. :O People that succeed with lower energy stones are just lucky. You can succeed with 0.1% but it's pure luck. Higher %, higher chance, simple as that. But once you get to +12, you get around 20%. Which means you have a 80% of failure every time.
There is luck involved but if you're persistent, you'll get it.
My advice would be to get your % as high as possible. You can do this by...
- Having a high guild armory (100%, level 10 preferred),
- Using % amulet (25% preferred),
- and having VIP (level 9+ preferred).
Level 9 VIP gives you around a 30% boost. With a level 10 league and a 25% amulet, you get close to a 25% success rate for +12. (Max level for VIP will increase to 12 eventually, so % may increase too, not including other benefits)
Of course some players prefer to just only one ES5 (or lower) for more chances but that's up to you. Both methods will get you results eventually. Hope this helped.