If you're worried about the spring being too long or binding....try this.
with the spring guide, spring and piston out of the rifle, place the spring guide point up on a table. Place the spring over it and then the piston. While guiding it with your off hand...push the piston down slowly and steadily. Try it with a "soft" stock spring first...to get the feel of it. (Don't lean your face over top of it in case you slip...d'uh. but i mention it for good reason
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The open end of the piston should end up flush with the table (the tabs of the spring guide might be in the way...but you get what I mean). Just shy of that is where it'll be when it's fully retracted (i.e. the most the spring will be compressed in normal usage). The inside of the piston shouldn't be binding on the spring as it compresses...it'll rub, but it shouldn't bind. Watch the end of the spring against the spring guide (especially if there's a thrust plate for a bearing setup). Make sure the very end of the spring isn't spreading out...if it is, it'll stop the piston from retracting enough. Never had that issue with Modify springs...not all bearing spring guides are exactly the same.
But if you have multiple good springs that it won't pull...I suspect the motor, or wiring. Try adjusting the motor a bit or swapping it out. Fully charged batteries of course.