You can almost see pieces of the puzzle start to move together on the table. A dozen people working for one year will produce far, far more information than one person working for many dozens of years. Gelatin is a huge part of the picture (Thank goodness there aren't a bunch of silver variables, too! God Bless silver nitrate.)
Back to Capt. Pizzighelli's observation: "while one sample of gelatine will stand digesting for fifteen minutes with ammonia at 70C, another will produce a foggy emulsion if a temperature of 50C be employed". It's not just a matter of temperature in isolation. One gelatin will likely be fine in one recipe and bad in another, even given the same temperature. And, even variations to a recipe; hrst's observation about fogging a sensitized emulsion is spot-on with things I've seen. This is why totalamateur's suggestion about sticking with one recipe in the beginning is such good advice, and why it's best to start on the low side of a particular recipe's temperature range. Having said that: if one recipe seems to fail consistently for you, there are enough very good, very simple published recipes now to try something different. Just because the big picture is complex, does in no way mean that making a basic emulsion is complex.
Does anyone know how to start a 'sticky' thread? I think we should start to make a list of specific observations of exact times and temperatures, exact brands of gelatin, etc, with illustrations. I have high hopes that some of the lab equipment geeks here (a term meant to be the highest accolade!) will invent any number and manner of meters, but in the meantime, just a plain old list of brand names and procedures will go a long way. Important information gets buried in a thread like this.