I have been using ahri for quite some time now. But I have notice in laning phases and roaming ahri is very good. Also escaping ganks that aren't wanted. But going into mid game late game I feel she just falls off. Im doing the most ap damage on my team. Even though I am fed or ahead. She still gets out scaled. is their any way to find out what is happening? Build I put on her is 
. Is that a bad build??
That's a decent build, but I would pass on the Randuin's. You really shouldn't be taking many shots as Ahri because she is very good at manuevering around the battle field and picking the right time to go in.
Instead, you should get something with more CDR. Assuming standard masteries/runes, you would have 15% with DFG and another 10% for when you have a blue buff. Picking up either an Athene's Unholy Grail or Morellonomicon will give you the last bit of CDR to cap you out and provide you the mana regen you'll need in the late game to give yourself so me more presence.
Ahri can still zero someone out with a full combo plus DFG in the late game and she's excellent for getting picks with a Charm. Did you know that Charm increases DFG damage too? Charm + DFG is a lot of damage when you follow up properly.
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Should I go with 
instead of randuins? I don't try and take too many shots with her. I normally look for who is out of place or who is alone. ult plus combo and get out. Now the charm with DFG? now do you charm then dfg? and finish the combo or charm combo dfg? what build do u prefer?
Ahri have base damage high but scale ratio is a bit bad. Just build her with CDR and/or lich bane(ahri's attack speed is good for lich bane).
