How carefully the paint is prepared for thinning is also an important step. I make sure first that the paint is at the proper room temp, then I remove the cap and stir it completely with a small stick or sprue piece. Shaking the bottle instead of stirring is not sufficient, many solids will be left settled at the bottom. Once the solids are off the bottom, I complete the mixing with the little Badger mixer, then thin as needed, again using the mixer.
I do find MM acrylics to be a bit inconsistent, but not very much. Like Tucohoward I do use whatever amount of thinner is needed, I think for me, thinner paint is better than thicker.
Is it possible that your attempts were with paint that was not thoroughly stirred and seemed too thin?
Just my thoughts.
Patrick