My experience is that the X7 won't spray your basic pancake-batter big box paint without serious thinning.
I used a 2 gallon bucket today causing hose to slightly bend being parallel with bucket, so tube screen is not being blockedMake sure the pick up hose / tube screen isn't blocked or touching the bottom of the bucket, either of which will not allow enough paint to be sucked up into the pump and therefore starving it of paint.
Might make sure your filtering/straining your paint.
You might not think it matters but I even strain fresh paint. Your 515 should do ok with some debris but your smaller tips will not like the smallest of junk in there.Also, my tips are constantly clogging?!