They play a factor. You're far more likely to know when you have a bad one than you'll notice having a good one. Training affects the physical attributes while coaching/playing time affects the learned skills. A lot depends on what sort of players you draft/sign and what you're trying to develop. But, in a simple world, I'd say 50% training, 20% playing time and 30% coaching. Although, if you have a bad coach, you'll see players decline. I'm not sure they decline if they just don't play(don't know for sure because I've never sat a real prospect for an entire season).