So on my last couple of brews I have noticed that yeast isn't just collecting at the bottom of the bottle, but also creating a thin layer on the side wall of the bottle. This has all been with the WLP023 Burton yeast, this is the first generation I harvested from a batch a few months ago. The beer tastes just fine, so I don't think something else has infiltrated my yeast stocks, though it has changed a little in behavior since I first started using it as well. On the first few batches, I would get a nice solid, even yeast cake but the last two have been very chunky. I assume the two changes are related in some way, maybe its because I have had the yeast in the fridge for a few months. I guess it doesn't much matter, as long as the beer tastes good.
I do use starters to revive thhe yeasties and even oxyclean has trouble cleaning the bottles!