I bought that exact same Army Painter set earlier this year. Unfortunately, the quality of the paints was all over the place. Black, white, and metallic paints worked well but every other paint was all over the place in quality.
Didn't matter how long I shook the paint, sometimes it came out of the dropper with too much medium and I couldn't paint with it, sometimes it needed little to no thinning and was great, and other times it was oddly thick. Felt like I was fighting with the paints most of the time.
Or maybe I just got shafted with an old, shitty set. Hell if I know.