Most hefeweizen style yeasts (WY3068, WLP300, WLP320 for example) are extremely vigorous top-fermenting yeasts. What you're seeing is the kreuzen, not the yeast cake. If you look at the bottom of the fermentation vessel (I guess you can only do this with clear vessels) there will be a thick yeast cake at the bottom. I've now done 5 batches with these yeasts and every time the kreuzen is extremely thick & healthy looking. There's nothing wrong with that kreuzen hanging around until that beer is ready to rack to secondary. If you get an equally thick kreuzen on your secondary, it's wasn't close enough to being done fermenting in primary.