I am playing this one song now that has a lead riff, and a hammer on and pull off on the D string. And the right to a note on the G string. Whenever I hammer on from fret 5 to fret 7, when it hammers on fret 7, I either miss the string or it makes no sound. Whats up?

i don't understand the question, it you missed a string the note isn't going to sound just because you had hoped it would.

Ok. I play a D note on the A string. Then immediately on the D string I do a hammer-on pull-off riff that goes from note G to note A and pack to note G. And immediatley after that I hit the D octave on the G string. This is the lead riff I play on Sliverchair's "Acid Rain". I know the real basline is simple, but I and the band think my riff is neater sounding than the Silverchair one. The tab would look like this.

G----------------7-------
D--------5-h7-p5-------------
A------5----------------
E-----------------------


the timing is pretty quick....

so, where I am getting stuck at sometimes is the hammer from fret 5 to 7, and when I get that right, I usually miss the pull off.

i guess just practice would help, but didn't you post something about this riff a couple of weeks ago?
make sure your hand is in a confortable position to do the riff, maybe if you play it in a bar chord position that might help. you got enough volume, strings to heavy.
i use heavy strings but they took a while to get used to.