Bonding Aquascape underwater

Question:

I bought some of the Aquascape epoxy, the coralline coloured stuff to do some coral mounting and bonding live rock together.  I followed the directions properly but for some reason i really couldn't get it to bond or hold and of the corals in there places and was hoping you could tell me what i did wrong. 

I had a small acro mounted to the rock on a flat plug and it will holding in place until i tried to mount my dendro  onto a ledge, i bumped the frag very lightly and off it fell. 

Then the dendro wouldnt mount i used enough to cover the bottom of the piece with about ½" thick part of epoxy, jammed it into the rock and about 15 mins later it had fallen, tried again a few times reworking the epoxy and then mushing it back in place but trying to get it to adhere and still no luck, finally the epoxy was to hard to work with and crumbling all over my sand bed so i gave up. 

I decided today that i would use the last of what i had about ¼ stick of each part and try it again, i had the same results. Same outcome, in the end ive wasted about half the epoxy trying to stick various frags.  I really don't think that where i was mounting it or the size was the problem, i think that i must have been doing something incorrectly.  the dendro is about 2-3"x1" and trying to mount it to a ledge that is about a 45* angle.  What else could i have tried to make it hold in place besides adding more and mushing it in place like i was doing?  The epoxy dried like rock so i know that i wasn't mixing it incorrectly. 

Thanks

Shane