It certainly appears that the model is suffering from lack of adhesion if the wheels slip. But not if the locomotive just simply stalls, then it is a lack of motor power. It would be interesting, if someone could be bothered doing the measurement, to know what the weight of the locomotive only section is, and how the weight is distributed over the six drivers, i.e. is it tail heavy? Depending on the answers to these questions, you can then decide on how to add and place additional weight.
Adhesion is also affected by the type of metal used for the driving wheel tyres, in that nickel silver is more slippery than steel for example. These days the best material for driving wheels is probably stainless steel.
However I would never advocate the use of Bullfrog Snot, it is a very poor method of increasing adhesion. You loose power collection on those wheels and increase the amount of rubbish pick up by the wheels.
The TOR 32 has Stainless steel wheels, & I find they tend to pick up a lot of black muck on the wheels more so than other loco's, also being polished until the wheels are worn in a bit tend to also be more prone to slipping than the other NS fitted wheels.
As they wear in they do improve in the pulling power, the problem is really getting the weight distributed evenly, & why I like getting the lead flashing under the keeper plate right across the length of the driving wheels, rather than just in a single spot such as the dome.
I understand that other than the Rclass, the only remaining steam loco's to be built at the old SDK factory is the 59cl, not sure about the 38cl reruns which will mean the 59's are likely to come with the same problems if it comes with plastic body, anyway like the 40cl the 59cl were quite prone to slipping so may be prototypical I guess, but will be also a pain for the modeller to overcome.
The use of the snot, is it not designed more for using on models that have traction tyres?