im having trouble going up the strings smoothly, like when i play warm up runs. im ok at E and A, but it starts getting harder when i go up to D and G. i noticed that my thumb is always stuck on the top half of the neck whenever a play the last 2 strings. is this the problem or is it something else?
hmm, what do you mean stuck on the top half?
if its what i think, try playing with your thumb on the center of the back of the neck. This will help playing anything if you can get it to be natural.
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Practice makes perfect, but heres a couple things to check for to make sure your getting the most fluid of motions

Thumb is always perpendicular to the fretboard
Hand is not "grasping" the fretboard
Fingers are spread out and not bunched up
Something my teacher tells me is that generally it's good to have fingers pointing up a bit. Not sure if this helps, but in case it does, there you go.
Your thumb should be on the center of the neck, so it acts as a pivot. If your fingers are long, you can get away with hanging the thumb over the neck like a guitarist, but if smaller, its going to be a hindrance.
top half means that my thumb seems to be stuck on the top of the neck... but i think keeping my thumb in the middle of the neck is the solution... will give it a try tomorrow... thanks guys!