Cast on Ailment is trash

No it's not.

https://poe2db.tw/Cast_on_Elemental_Ailment

Literally says on skill itself:
1. Gains 10 Energy per Power of enemies you Freeze with Hits from Skills.

This means it doesn't matter what is your damage, you freeze mob - you get 10 energy per enemy power, so freezing normal mob give 10 energy, magic - 20 energy, rare - 50 energy and unique - 200 energy.

Since you can't Freeze often, CoA is not good for single target if you only do Cold Damage (And apply Freeze).

2. Gains 1 Energy per Power of enemies you Shock with Hits from Skills

Same thing as Freeze, your damage doesn't matter, you apply shock - you get 1 energy. But you can reapply Shock while target already Shocked, so you keep getting energy.

It's ofc better than Freeze for Single target.

3. Gains 1 Energy per Power of enemies you Ignite with Hits from Skills, modified by the percentage of the enemy's Ailment Threshold the Ignite will deal

And ONLY Energy you get by Igniting is affected by your Damage. While its more important that you have enough damage to apply Ignite with each hit, which will generate a lot of Energy because you can apply any amount of Ignites on target.

Crit or more damage does help, not by getting more energy directly, but to apply ailments more often/more consistently which does increase amount of energy you get. But as long as you have enough damage to apply ailments consistently, further damage increase will have no effect on energy gain.

And possible reasons you generate low energy:
- Elemental Focus support that fully disable Ailments
Single target:
- Low Fire Damage/Ignite Magnitude and low amount of Ignites Applied
- Low Shock Chance and not frequent Shock reapply
For clearing:
- Low Cold Damage, so mobs die before they get frozen so you not get that 10 energy.

If build has guaranteed shock on every hit, only way to get more energy is more frequent hits.