I can do it only for the top string. I have problem tremelo picking on any string under it. The pick just gets stuck. I pick at a 45 degree angle, it still gets stuck. I've tried keeping at it, hoping I learn in time and it won't get stuck later, but it still gets stuck. When I pick the top string it doesn't get stuck. It's really smooth. But on all the lower strings, when I try to tremelo pick it gets stuck.
Sounds like your downstroke is going too far, that's why you can only do it on the top string. Practice on the other strings, and slow down. You're not going to play it fast and correct until you slow it down.
If you have a mirror that you can move to your practice room, it can be a good idea to set it up in front of your picking arm/hand. This will allow you to notice things about your picking that are quite hard to see normally.

If you set the mirror up at the right angle, you'll be able to see exactly how far the pick moves each time you do a pick stroke. You can then adjust the size of your pick strokes based on the feedback you're getting from the mirror. (My guess is that your pick motions are way too big).

You haven't mentioned whether you pick mainly from the wrist or arm. For fast tremolo picking, I definitely find wrist motion much more economical and easier to control.
get Dunlop jazz 3 picks .... just keep doing it and doing it ... practice makes perfect .. youll get it ... dont over extend your muscles either
Google Paul Gilbert pick drill
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Choke up on the pick a little bit. If there is too much of the tip exposed it will tend to catch on the strings easier. In order to tremelo pick fast, your pick must be barely hitting the strings -- and make sure the motion is as tight as possible using only your wrist for movement. You should be only moving your hand enough to play the notes -- any further and you are wasting time and energy, requiring you to tense up which is the wrong way to go about it.

Also, every string has a different feel. You should practice just trem picking one note repeatedly on each string to get used to how the feel changes. It can be helpful also to trem pick over scales with which you're very familiar just as a way to get used to transitioning from string to string and maintaining the sound over all of them.
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Pick should be perpendicular to the body of the guitar so you get the same attack up and down.

Set the metronome to 60 and just alternate pick 8th notes on each string for a few minutes a day, then work your way up.

Tremolo picking isn't a short cut to fast playing, it's name is more for the rhythmic device than the technique. http://en.wikipedia.org/wiki/Tremolo
Use a thinner pick, It should help as it would glide over the string for a little more control. I had trouble with the same thing. :/
I dunno, you could check out Guthrie Govan's guitarworld instructional on alternate picking on youtube. He's bound to give you some good information (although he can be a little vague)..
I had trouble with this but got used to it, It really improved my playing once i got it! Just do scales slowly doing up and down picking, and eventualy get faster thats how I did it!
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