There are three things you have to do to use Andrea/Vallejo paints. First, shake the bottle and don't be afraid to tap it hard on the heel of your hand. Next, shake it even harder and longer and smack it on the heel of your hand. Finally, shake the bejeziz out of it and whack it on the heel of your hand.
The paint and carrier will separate during storage, and if you had them stored a long time, it will take longer to get the elements back into working order, The paint plug at the nozzle is considered to be a good thing, since it reduces air getting at the paint, increasing the shelf life.
Once you get used to the little anomalies, Go here for an excellent free tutorial on this paint. http://www.acrylicosvallejo.com/ Click on the Miniature and model color link and scroll down.