Why isn't the paint adhering?

I'm very old (I remember the Beatles on Ed Sullivan) and very recently have
gotten back into modeling. I've been playing around painting 1/35 scale figures
and have been using MODEL MASTER paints exclusively. Some of the paints
brush on very well and have a "rich" look, but others (like signalbraun) come out of the bottle thin and are very difficult to apply, taking many coats
to make an impression, and in the end look washed out. Is this just a characteristic of the particular paint , is the paint too old, or is my technique
off?

THANKS donrenner