If you find that you press at an angle, try turning your trumpet to an angle so that you are pressing straight down relative to the horn, if that makes sense.
I do it too, its just how I've always played.
Incline your horn to the right just a little bit if you press at an angle. Especially if you have small hands (which i dont, but it still works for me)
If you have small hands, your fingers may not reach up all the way to the top of the valves.

Wait a minute.
You said it goes slowly, doesnt get stuck in one place right?
Then it is most likely needing to be broken and and oiled.
just sit there, oil them, and press them repidly for a minute or two.
That will get them broken in pretty quickly.
I remember when i first got my horn (a stradivarius) they used to stick quite often, and id have to oil thme a lot.
Now, after about 6 mothns of owning it, i dont have to oil near as much.