Production / Mixing can be difficult when you are so close to a project. If you are extremely aware of each and every part, your mind can play tricks on you. That ambient part can be mixed way too low because your mind knows it is there and tends to make compensations for any deficiencies (volume, separation, etc). Or maybe you think that part is so cool that you tend to mix it too loud to make sure everybody hears it.
Nothing like doing a mix, shelving it, coming back 2 weeks to 2 years and listening again. There is always a realization that one or two parts are way too prominent.
You need fresh ears all the time if you don't want to let your mind's ear deceive you.