Im thinking you might just need a few more coats. Some colors don't cover as well as others but SW endurance is good quality stuff so I am a little surprised. I remember painting a bedroom red years ago and it look 5-6 coats before we had uniform color.
Priming probably wouldve helped but I'm not sure how much. My family came her from Italy in the 60s. All worked as painters, landscapers, or masons. My old man was a painter for 40+ years. He only used primer on new drywall and even then, I can't remember how many times we used regular paint because we ran out of primer and didn't want to run out to the paint store.
What kind of finish are you using (flat, matte, eggshell, etc?). If you're using anything shinier than matte, you can't touch up the paint after its dried without it being very noticeable. So I
For example if a wall was dry and you noticed a spot that didn't cover well and you touch it up with a roller, it'll be very noticeable.