[lynxrpken]

well I am not going to give a lot before I see the actual pics but some common mistakes people I know have made are.

painting in order like painting the model from the bottom up
inapproprate inks
no order
basing late

lets go from the top when painting minis I find it easiest to paint from the deepest point of the model out. for a cadian this means paint the face then hands then uniform then boots then impearial cimbols and vox and gun. this makes it easy to get all the details in the model brfore working on the next piece. it is easier to paint a helmet without touching the face than visa versa.

use a ink that is one or two shades darker than the color you are inking. this gives close to the dry brushing look with litterally 10% of the effort. so camo green base catachin green ink. they do not make that ink so delute the paint in water by dipping the brush in the paint then the water knocking off the excess paint and more water and then dip the brush on a napkin more water and apply to model dry the brush and brush off the excess water paint from the model. all you should be left with is paint in the groves which makes the lighter color POP out at you.
the one exception to this is that most shades of grey work best with a chaos black wash and white looks best washed with codex grey unless the primary color of the army is blue then I would use shadow grey over white.

paint everything the same color. lasguns all look the same in the srmy every ones base armour is the same color. anything else looks tacky ( my oppinion )

if you are going to do up the bases ( highly recomended ) do it before the models are on it. I just got this idea myelf and it takes some practice not to frost the mini but it is so worth it. It comes out a LOT better.

pics would help more if this is not helpful. hard to say what the problem is with the painting without seeing the painting.

hope this helped