I know that saddle well since I’ve owned one for 8 years and put it on many different horses. It was the saddle that launched my obsession with saddles. 
Anyway, I’ve seen this before, and it could be a couple of things. Alas, without being there with you, it would be hard to say which:
–It could need a flocking adjustment. Since Amerigos are flocked with a synthetic blend, it can take them much longer to “settle” compared to a saddle flocked with 100% virgin wool. Virgin-wool saddles can settle in 6 to 8 months, or even 2-3 months if they’re in heavy duty use. But my Amerigo took nearly 4 years before it needed an overflock! Adding flocking to the front area, in front of the untacked middle portion of the panels, made a huge difference for my saddle’s slippage and the saddle’s overall balance.
–Try girthing it on different billets. On many horses, I girth mine on the second and fourth billets, and that significantly reduced slippage. Amerigo is designed to sit pretty forward on the shoulder–basically the exact opposite of many other brands–and girthing it on the back billets helps make that happen. (Occasionally, I do run into a horse that’s better served by a different billet combo.)
–It could be way too narrow or way too curvy for the horses you’re putting it on. Since you said this happens on many horses, this is probably not your problem, but it’s the most common reason that I see Amerigos slipping back on horses owned by amateurs.
Barrington Saddlery carried Amerigo for several years, although they don’t anymore. So maybe their saddlers (Kate Briggs and Michael Dainton) could shed some light on your situation. If not, Ann Forrest at Equestrian Imports is the person I usually send folks to re: Amerigo questions. She’s an excellent, well-qualified fitter who has carried the Amerigo lineup for years, and she’s used to working at a distance.
Good luck!
