poorly formed cherry fan

[Ken the Chef]

I have been training a cherry fan for three years.

The left side has made plenty of branches and has a good shape while the right has only formed two main branches to form a “Y”shape off the main stem .

Can anybody advise how to encourage the right side to produce more side branches to catch up with the left.

[FB.]

Look up "nicking" and "notching".

It is basically where you make a small slit in the bark above a bud that you want to cause to sprout (or a small slit in the bark below a bud that you want to prevent from sprouting).
The damage to the bark affects sapflow and growth hormone transport, which then causes them to accumulate above or below the damaged point.

Higher branches in a tree are the dominant ones and they send growth-inhibitors down to suppress lower branches. By preventing sap from coming from above a bud, the bud will grow. By preventing sap from coming from below a bud, its nutrient suply from the roots is reduced and the growth inhibitors from higher branches tend to accumulate at the damaged point and suppress the bud just above the damage.