Uneven growth in front beds...

I have 2 front beds, one on either side of the porch steps. There is also a very large maple tree in our front yard (which DH refuses to have cut down because it's too $$$). The beds have identical planting schemes, but the one that is farther from the tree does really well, and the other bed's plants are stunted. This happens every year, so last year at the end of summer, I dug out the entire bed 12" deep, cutting/hacking out any tree roots I found--some were as big around as my arm!. I figured that would eliminate the lopsided growth between the 2 beds. I am very dismayed that my theory was wrong, and all that hard work seems to have been for nothing. The beds are again lopsided. In the fall, I planted bulbs and those seemed to come up fairly evenly this spring. The boxwood also seem to be pretty equal. But the dahlias and morning glories I planted recently look very different. The one bed is flourishing, the other floundering. I am pretty sure they get the same amount of sun (or close to it). I will have to find out for sure. Is there any other explanation for what could be going on?