Pro-Mix contains 14% Perlite, whereas you are lucky to find any measurable amount in Miracle-Gro Potting Mix. I went through the same thing with my EarthBox the second year, where the Potting Mix had all compacted down, and it was like Play-Dough when I emptied it out. Simply put, you are not getting aeration down to the root system now, hence the yellowing and poor growth.
My recommendation is to as a minimum, add in Perlite to the MG at a 4 part MG, to 1 part Perlite ratio. If you really want to tweak the Combo Mix, get some Decorative Bark Groundcover at Home Depot (or use Reptile Bark at a pet store if you only need a small quantity) and do the 3:2:1 Combo Mix of Potting Mix, Bark, and Perlite. I am using this in all of my EarthTainers and EarthBox this Season with great results.
Raybo