Insufficient Beauty
In "Diamond," "Pearl," "Platinum," "HeartGold," "SoulSilver," "Ruby," "Sapphire" and "Emerald" games, Feebas only evolves if its Beauty stat is maxed out. You can raise this stat by feeding it Pokeblocks or Poffins made from Berries. Pomtre Berries and Gamlon berries work the best. Entering it in beauty contests and winning will also raise this statistic. Once Beauty is above 200, Feebas is ready to evolve.
Needs a Level
Once Feebas's Beauty has reached an appropriate level, it still must level up before it can evolve. You can use it in battle ̵1; though its moveset is poor, it can learn Technical Machines ̵1; or just feed it a Rare Candy to raise its level by one. Any Pokemon in battle will have a share of the experience even if a different Pokemon strikes the final blow on the enemy, so you can easily leave it in battle only one turn before switching to a better one.
Evolving Feebas in "Black" and "White" Versions
Nintendo simplified the Feebas evolution process in the fifth generation of "Pokemon" games. Simply have Feebas hold the item "Prism Scale" and trade it to another game to have Feebas evolve into Milotic. The Prism Scale can be found on Route 13 or in Undella Town.
Final Warnings
If a Feebas is fed too many Pokeblocks or Poffins, it will become "full" and refuse to eat more, so do not feed it extraneous blocks. Feebas who have a Modest, Mild, Rash or Quiet personality will show the strongest response to berries that raise beauty. Beauty remains constant between 3rd and 4th generation games, so you can trade and still have the Pokemon evolve. Finally, in "HeartGold" and "SoulSilver," massages from Pallet Town's Daisy or the Haircut Brothers in Goldenrod City also raise the beauty stat.