The roller picks up the adhesive from a tray, then depending on the machine, the adhesive will travel directly to the back of the wallcovering, or to another roller before it reaches the wallcovering. If you have a product that doesn't break down with agitation, you may have problems using it in a machine. Peters advised first and foremost that paperhangers read adhesive labels carefully. But thinning is risky, warned Kay Peters. But I know people are going to, so I'd say add as little as possible.
Most problems come from adding too much water and diluting the solids of the product.
Then they have trouble getting the paste to work. You shouldn't get one glop and have to wait one or one and a half minutes for the next glop. Bob Kelly recommended mixing the adhesive according to the manufacturer's instructions and pasting a piece of wallcovering by hand. If it looks like it's the right viscosity, then you can dump it into the machine. The difficulty is that once you dump it in, it's in there.