I'm assuming (though I could be wrong) is that what you're currently doing is trying to lift your finger off the string at right angles to the fret board. This will result in a quieter second note like you described. A correct pull off is a bit more of an active process. Try pulling down (towards the floor) a bit more while you are pulling off (almost plucking the string with your fretting finger). This will probably seem strange, but you'll get a second note which is as loud as the first. With practice you'll find the amount of pull down that you have to do to make the second note ring out.

Some exercises you can try are - fret the fifth fret of the high e string (the thinnest string) with your 1st finger then fret the 7th fret with your 3rd finger. Hit the string and pull off to the 1st finger. Once you have managed that with no loss of volume then hammer back on with the 3rd finger then pull off, hammer on etc.. until you can get a constant steam of notes without having to hit the string again. You should try doing this using different fingers in different combinations on different strings in different positions of the fret board. This idea is the basis for a lot of guitar exercises and time spent doing this is well worth it (you should try and spend some time on this every time you pick up the guitar.)
Hope this helps. If not then give us a shout.
AndyP