The first thing that comes to mind for me is that you may have had too big/thick of a clump tied. You want it 'thin' enough that the moss touching the wood/rock gets light so it can grow and thus attach itself.
When I first got some I just sat in on my piece of driftwood. The edges caught on but the bulk in the center -never- did, even after over a year. (Yes, I let it sit there that long and it eventually turned into an expansive blob that I have only recently started pulling apart.)