It seems that some yeast strains are easier to wash than others. It appears that some strains actually settle underneath the trub but most settle on top. I've had some lager yeasts that weren't as easy to wash and the Fullers strain (Wyeast 1968 or WLP002) makes flocs so big they often settle faster than the trub (even after a lot of agitation to try and break up the flocs).
What do you guys do for these difficult-to-wash strains? I usually just accept the trub; I only wash it once anyway and I always plan another batch within 48 hours of the wash. I can't yet blame it on poor washing or excess trub but there have been a few repitch batches where the head retention was noticably poor. It's also possible I over-pitched those batches.