What Mixxer5 said; only if you cant raise any that means that you'r retinue cap has either been reached, or is not high enough to deploy any retinues. If you hold your mouse over the retinue army you can see how much of the retinue cap it uses. The way you increase the retinue cap is either by developing your provinces or gaining more provinces. If you mouse over the retinue cap it should give you further details.