Whoa, what a whopping post! The models look nice. Some of the pictures are a little too far away or too close up, but good none the less.
Its really hard to tell from the photos, but it looks like you could use a bit of wash or ink. Especially on the bone areas and large sections of flesh, throw a bit of a brown wash/ink on top and watch the detail come out. Also, do you water your paints down?
Hmm washing your brushes isn't quite the same... GW have a line of paints called washes. The most common example is devlan mud. They are pretty much really heavily watered down paints, like almost the consistency of water. There is in agent in them though that causes them to spread out and provide even coverage. These are the easiest way to apply a wash, you can use them straight from the pot and use a standard brush to just slop them onto the areas you want them on. They will darken the skin a bit, but they will also add much more depth to the shadows. The other alternative is inks, which GW produced before washes and several other companies produce if you use different lines. They are slightly different, a little bit thicker consistency and a much higher pigment. They also do not flow evenly on the surface, they like to head straight for the cracks and deep areas and really darken them in addition to somewhat staining the entire surface. The other property of inks is they come out glossy while washes are matt finish. Inks do need to be watered down. Both are similar in their use but give different results. Different people like one or the other, I tend to use a mixture. They will make your models look so much better and with very little effort. About watering down your paints, don't use them straight from the pot or they go on too thick and obscure detail. Put them on a palette (or old plate, jar lid, ice cream container lid whatever) and add a bit of water to them. It may take an extra coat, but two thin coats looks far better than 1 thick coat.