The chance to get the better potions is better in higher levels. I think in lvl 15 the drop of a potion is guaranteed, you don't get the stupid XP-brew instead.

What stage are you farming? I've been potion farming on an alt account and I was surprised to have Superior pots drop more than I expected on Stage 12. That being said though, you still gotta strive to farm the highest stage you can, Stage 15 if possible. 

actually even at higher there is no guarantee that you will get potions, I kept on getting 1 potion even at level 6 , in fact I keep getting lvl 1 potion at level 10 and sometimes i get 4 potions at lvl  1, game is frustrating that way. but this is not by design, most likely the programmers got the logic wrong and it does not make any sense. normally programmers must have thought things through, because if they did not , then it would mean they just wanted to annoy everyone. lol

When  it  comes  to  potions,  your  best  best  bet  for  the  superior  variety  is  to  farm  stage  10  or  11,  dont  ask  me  why  but  i  very  often  get  back  to  back  superior  potions  x2  on  these  levels.  It  has  happened  every  time  i  do  it  so  its  not  just  good  luck,  they  drop  more  on  these  stages.  

So basically, you need an already strong team to get potions. Otherwise, you just farm crap, and your champs stay one star, max 2 stars ascended at best.... Who said russians don't know how to make games?? :)