Wel i hardly call it a problem, but yes i experience the same with Taxiphyllum moss sp. and others.. In some cases it does attach in others it doesn't.
Also the moss ID is always kinda tricky and confusing and in many cases a best guess in a yes and no discussion, than all you can do is open a pol and go with the majority and hope they are correct. As i personaly experienced buying alledged Christmas moss from a specialized moss web shop named
www.aquamoos.de . And after posting pics of it several members doubted it was Christmas moss. Same story, i think i once bought peacock moss and growing a lot of it at the time, quite a lot, lately sold a batch and the buyer repleyed "Sorry mate, looks good but that aint Peacock moss!"
I'm reluctant to go into the discussion and i do not argue with the label.. It is moss that i know.. best guess the family name is correct it a Taxiphyllum sp. and the other one a Vesicularia sp. and i leave it at that.
But i see both sp. attach and also grow and branch freely around.
Tho i do notice that the provided substrate to attach to definitively plays a role. For example softer old woods and or rougher rock surfaces do beter. Than spiderwood is rather smooth and young wood type compared to Mopani that is rather older and softer at the surface and more rougher shaped. Or a pebble stone is much smoother than a piece of lava rock. Mosses do not grow a rootstructure as plants do, it only grows Rhiziods (tiny hair roots). Try to imagine, it doesn't attach in form of a suction cup, it attaches with growing a hair root into a tiny cranny in the provided substrate and this rhizoid also expands grows thicker and wedges itself into the substrates cranny. This all is on tiny microscopic level.. It's the smoothness of the substrate that provides the foothold, emersed moss even will attach to glass, but not as firm as to a lava rock.
But.. Than if a moss is placed submersed and even to it sinks, still bouyancy has effect on it it doesn't succumb to gravity. If it has no need for a substrate to attach to and to take nutrients from because it gets this from the water column than why should it put energy in attaching? Than everything that grows and branches away from that substrate due to the bouyancy of the water it keeps growing and branching freely in that direction. Moss parts that grow in the direction of the substrate and that are puched against it finaly will attach as explained above with growing rhizoids into the crannies.
Than a logical approach looking at the type off moss.. Taxiphyllum and Vesicularia are branching moss sp. than if you want large ereas to be attached with this moss you have to push as much as possible of this moss against the substrate you want to grow it to. To give the rhizoids a change to grow into the crannies. If you leave it to its own devices it will branch in all directions most likely the path of least resistance of which you have little controll.
If you want more control with the least effort than use gravity in form of a drystart. Than it's forced to search for nutrients in and on the provided substrate. It doesn't find this nutrients in the air, thus it will grow towards and creeps over the substrate where ever it is verticaly or horizontaly and grow its rhiziods into it and attach. Once you are satisfied with the way it spreaded, flood it and keep trimming all off that branches away from the substrate to much.
If you want to do this all submersed you have to push it mechanicaly on the substrate to make it attach.
Another logical approach.. Just think of it, all mosses grow rhizoids, it attaches by growing and exanding these into the crannies of the substrate.. Thus simply if you force the moss against a substrate with sufficient crannies so the rhiziods can grow into them all mosses will finaly attach. How firmly it attaches depends on the smoothness of the substrate surface.