You should use more yeast for a big beer. Letting it sit longer doesn't necessarily bring more fermentation, as the yeast only has so much to give. You an google the Mr Malty calculator for amounts.
Some yeast are more flocculent than others, that is, they fall to the bottom of the fermenter. It doesn't mean they ferment poorly. Flocculation is just another characteristic of yeast. No idea why some do it better than others.