Hi Edgar, there are a couple of options you can try to solve this.
1.Way is to phase invert the channel if you are working with just a stereo track and dont have access to the individual parts thats causing the problem.
2.Try a utility like mono maker or something similar that is able to reduce the stereo field of the instrument thats causing the problem.
Basically, out of phase means the left and right channel are cancelling each other out which makes it less mono compatible. Its not gonna sound good on mono systems eg radio/club system and tend to sound weird due to its out of phase character.
From my own experiences, i find that its usually the lower register frequency that are more prone to going out of phase if you are not careful. I had a mix in the past that has a slight phase issue. Was at Jesse Studio and he showed me how to solve and minimize phase issue so i think he's probably the best person to explain in detail on this.
Hope this help in the meantime and good luck

ed