Enhancing is a very tedious task and you can get hammered by RNG where it goes from +19 to +9 several times. Don’t panic, this is just RNG.

Enhancements stones are used prior to reaching +20 on equipment.

1-5100% to level up.
6-9Chance decreases, but level can only go up.
10-20Chance decreases, levels can go up or down.
I – VChance greatly decreased, levels can go up or down.

Enhancement crystals are found in Torment 3, boosted modes.

Enhancement CrystalLevel
ITorment 3, boosted level 1-10
IITorment 3, boosted level 11-20
IIITorment 3, boosted level 21-30
IVTorment 3, boosted level 31-40
VTorment 3, boosted level 41-50