Hmmmm, hard one this.
One of the reasons might be that the primer you are using is too pigment heavy(the finish of the spray is grainy).
Another one is the thickness of the paint you are using. Taking off paint with a tissue to do blending or layering isn\'t going to work, because you take the moisture away that makes your paint flow on smoothly. This technique is fine for drybrushing, but if you want a smooth finish, that\'s not the way to go.
Another reason for rough texture on painting and colours not properly flowing into eachother is the size of brush you are using. If your brush is too small for the area you\'re painting, your end result will only look smudged and muddy. As a guide, I use a 0 brush for most of my larger areas(i.e. jackets, cloaks etc.) A 00 for most of my smaller areas(face, pouches) and a 000 for edging and eyes and stuff.
End of the day, if your paint flows with the consistency of milk/cream, you should be alright if you keep the above suggestions in mind. Good luck, and it does take practise...unfortunately