Cannot get the GWR 5700 Pannier loco to work. Whatever I do, I lose steam.

Tried it out as i'd never driven it before on the first mission, taking 4 carriages up from kingswear. Used to a coronation class, i went full steam ahead... (literally). Barely made it to a quarter of the way up. Fair enough, I wasn't managing steam, try 2.

This time, being super careful about trying to keep steam up, I am v careful with the reverser and (don't know proper term) ?power lever?. Try to keep it above 180 at all costs. On the flat this is easy enough. But on the hill, even with reverser at the min it goes to of 15% and the ?power lever? at 15%, it was losing steam. And we were still losing speed against the 1 in 66 hill. Until we got down to 5mph and I realised it was going to stop unless we upped power. It took 100% power and about 45% on the reverser to get it to keep going at 5mph but of course this caused pressure to drop by almost 1 psi per second, obviously unsustainable. So I come to full stop, shovel some coal in and wait for heat to increase, pressure to build back up to set off again.... nope. Seems like once you get too low steam pressure, it just seeps out. When I came back to it after tabbing out for 10 mins, I found it had got down to 50psi, despite having firebox closed and loads of coal to in theory raise the pressure.

Okay, try number three. Sit in the station to raise the pressure ready for the climb. Firebox at 45% so that needs to improve, right!?? Shovel up to about 90% full. Wait. PSI goes down...what!??? I am now well and truly baffled. Surely a loco SITTING STILL ON A LEVEL PIECE OF TRACK should be able to increase steam - how did it get up to 190psi in the first place? (joking, ik it's set up by the scenario).

So how the heck do I get this thing to work. Is it broken/ bugged in how it loses steam? Or am I being a real noob and missing something?

Also loads of other forums say "look at the manual", well I can't find a manual for this thing anywhere. DTG seem to only host train sim world manuals these days. Several links I checked were broken.