I had the same problem.
I noticed that new growth will attach better and faster.
At first I just took pinches of moss, squished them against the rocks, and tied down the thickest, oldest parts. The tips were left free floating. It never attached, and when the thread rotted, it all came appart.
What I did later, which took a full sunday for a couple of rocks and one piece of wood, was to lie individual pieces of moss (christmass tree like triangles) as flat as possible on the rock, and tie them down making sure the growing tips touched it. Did this until the whole rocks were covered, makin sure to minimize overlap. It is very easy if you put the moss on the rock/wood, and squirt a little water with a syringe, it will flatten itself. Keep everything moist all the time.
After only three weeks, the thread came appart. The new growth was firmly attached, so very carefully I trimmed the old unattached moss. This left the rock like 60% uncovered, but the moss is growing beautifully in very flat creeping triangular patterns.
Hope this helps.
BTW: Low light, no CO2, no fertilization shrimp tank. It only took 3 weeks, maybe in a high tech it will take less.