Help with setting enchantment levels to an item

I've made a toolset that when one of the tools from the toolset breaks a block it will run the procedure "Add Enchantment [Efficiency] with level [Random integer between min: 1 and max: 12 included] to Provided Itemstack," and the intention is that each time the tool breaks a block it will get a random level of the enchantment "Efficiency."

This doesn't seem to work because if you add an enchantment level that is lower to enchantment level that is higher the enchantment level that is higher will remain the same, so the enchantment level of the tool eventually gets stuck at 12.

How could I get around this so that the original intention of the procedure is fulfilled?