If you don't do at least the full minute you will tend to get lower than actual readings. There is no point in testing if you won't do it right.
As all the others have already said, nitrates are best controlled with water changes but plants can go a long way to controlling nitrates completely. I have 2 of my planted tanks that I add nitrates to for plant food, the fish do not produce enough to feed them. They are rather heavily planted but they need more nitrogen than the fish provide. That could be why you only see 10 ppm no matter what.