Make sure that your a-bar is aligned correctly and is not too high or too low or off center, which will greatly affect how your needle rides in the tube. Since you said that you tuned your machines, I assume this means you were adjusting the springs, and thus the a-bar. The spring/ a-bar setup can be a finicky thing if it is just a little bit off, and when you adjust one, it will affect the other. You might want to invest in an alignment tool, which are cheap( pulse has one for maybe $10 bucks) which will get you the perfect alignment of your set-up every time. Besides the spraying, poor alignment will cause your machine to run less efficiently as far as duty cycle, follow through, and cps/hrtz is concerned. If the springs/ a-bar are out of whack, you might need more power to achieve the same results, which will cause your machine to run faster or hit harder than desired, coils to get hot....etc.
It is all relative, and every part of the machine works together in harmony to make it run smoothly and efficiently. It is sort of an equation or values, and if one value is incorrect, the whole equation falls apart. Also, if you are playing around with different spring set-ups to see what works best for a certain machine, a spring alignment tool is also a worthy investment. Pulse also sells one for $25 that is pretty foolproof, ensuring that your springs and a-bar are all in agreement and centered perfectly straight.
Adjusting the rubber bands may help, as well as putting a little bend on the needle bar. Ideally, the needle should ride the bottom of the tube, without being overly forced into it, which will cause excess friction, alter the performance in your machine, and cause lighter colors to get muddy from the metal on metal grinding tiny bits of steel dust into your ink. If the needle is gently resting on the bottom of the tube, it will be stablized in it's movement, resulting a smooth, predictable action with each cycle. Making sure that your spring and a-bar are set up properly with the geometry of the rest of the machine is crucial to achieving this.