I'm a bit of a matt enamel dinosaur and I have noticed several problems when replacing some of my Humbrol paints.
One problem is the extreme thickness of some of the paints as you mentioned. I bought some Humbrol Enamel Thinners and a glass eye dropper from the chemist (a plastic dropper might be melted by the thinners) and this does a pretty good job of thinning the paint or retrieving those that have partially dried out. It doesn't affect the matt finish.http://www.airfix.com/uk-en/shop/paints-accessories/enamel-thinners-125ml-bottle.html
Another problem is that some of the colours in new tins just don't match the old ones. I've had to mix white paint into a couple of cans that were just way too dark.
Humbrol used to have excellent metallic paints. When you used their silver it gave a result that looked like a sheet of polished silver. And dry brushing this with gunmetal gave a very pleasing result on armour. When I had to replace my silver paint I bought four or five cans trying to find one that matched the old stuff. Some looked like metallic grey, some looked like not so metallic grey and some looked like glaziers putty. Eventually I bought a tin of Revell silver.
When replacing my brass paint, which previously was very good, I got a can filled mostly with a putrid green solvent and very little metallic pigment. It doesn't work well at all but I might have just got a bad batch.
As for your transparent yellow paint. it might, as BB says, be just a bad batch with too little pigment, like my brass paint above.
If the matt paint is also drying shiny it's a mixing problem, the solids are not fully mixed with the solvent. I have a rule for Humbrol enamels, stir with a cocktail stick until the solid lump of pigment picked up from the bottom of the can has disappeared from the stick. continue stirring for a ridiculously long time that an obsessive compulsive would find excessive. Then continue stirring for half as long again. If you're lucky you'll have enough time left to paint something.
If it's just the way this colour paint is, and I believe that in any kind of paint yellow pigments are just more transparent than other colours, then I can only suggest a white matte primer or gesso to combat the transparency.
I like the look of matt enamels, and Humbrol have a great range of colours, They even have a paint specifically for painting Brown Bess musket stocks! But it would be good if they could manage their quality control so that we got the same product each time we buy.