I would look at differences in the amount of sun/shade and temperature (microclimate) first. If you still suspect it is soil related you might want to get a soil test, better yet a separate test for the two different areas. Some people will likely say Epsom salts helps (that could be relevant if the soil lacked magnesium but extra magnesium when it is already adequate in the soil naturally shouldn't make any difference) others might say soil pH makes a difference, but I don't know of any blanket recommendation as a one size fits all.
