Yes, that's the problem. Some species of bryophytes used in the hobby such as Fissidens fontinalis, Riccardia chamaedryfolia, Fontinalis antipyretica, Leptodyctium riparium, etc. as well as dozens or closely related species can be found in the wild very close to our homes, but it doesn't mean they can adapt to our aquarium. I have tried 4 or 5 species to the moment found on local rivers (with warm water) and I do fail every time... I guess Tropica and other brands not only multiply the plant material (which is for sure the easiest part) but they also work very hard on selecting nice, stable and adaptable varieties. I would bet that temperature is critical for mosses. Even in the warm area where I live, water rarely goes over 20ºC and is probably around 12ºC most part of the year. For example, for a Fontinalis local population grown in tropical waters it shouldn't be a problem to live between 20-26ºC, but even for a Mediterranean Fontinalis living in warmer areas than in the UK, living at 25ºC the whole year can be probably a problem.
An added constraint with mosses is the high variability of the plant material (according to the linked guide, aquatic bryophytes are especially variable due to the different conditions where they grow) but also the low reliability regarding material identification... this hobby lead me to mosses and I guess from on now it will be another "toy" for me. I am beginning to identify what I find but it is really difficult, even having some skills for being a taxonomist in the past and having local identification manuals.
Jordi