Don't try to think of the shielding effect as a complex attribution of an atom. The shielding effect is an attribution of a subshell. The s subshell has the best shielding attributions and the f subshell has the worst.
So try to think of it this way: electrons in the f subshell do not shield the increased nuclear charge as much as an s subshell would. And because of this the nuclear charge can affect electrons on outer layers. This I think is because of the shape of the subshell. You know s subshell is a sphere, p is three dumbell shaped things, etc. So they start out with a sphere covering the nucleus. Three dumbells can't really cover a spheric nucleus that well so the shielding effect is reduces. And as you move along the shape covers less of the still spheric nucleus, therefore allowing the nuclear charge to have its effect on outer electrons.
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