As far as I know we have a fixed chance on level ups in harvesting green, yellow and orange materials.

Instead of a fixed there should be a mechanic that increases your chance to level up a proffession.

For example;

For yellow materials, lets says a 30% base chance to level up, but every unsuccessful level up in harvesting increases this chance by 5%.

This at least the RNG can be a little less frustrating after the nerf.