Inconsistent fermentation issue

[johnny17]

The last couple fermentations of our IPA (15°P) using our first gen yeast (WLP090) have not attenuated as well as usual and I was wondering if anyone has any advice. Our original process was to brew with our 15BBL system into our 15BBL FV's. We would prop up the yeast the night before with ~35L of 10°P wort. Doing this we consistently had an AE of ~2.5°P in around 7 days. We recently brought in 30BBL tanks and have been brewing twice on concurrent days to fill them using a 15BBL yeast pitch (the first brew effectively becomes a starter for the 2nd). When we are using slurry from another batch this has produced similar results as before. The last two batches have been 1st gen's propped up as I mentioned before and the fermentations have been sub-standard. The first of these stopped at 3°P and the most recent one seems to have stopped at 3.5°P. The cell counts on the first batches have been a bit low, but not alarmingly so (~500,000 cells/ml/°P) and 24 hours later the yeast has roughly tripled its population which should be sufficient for the 2nd brew. I am fairly certain it is not a nutrient or oxygen issue as we provide ample amounts of both and the lower gravity brews we brew in this way have fermented very consistently. There are a couple things I am thinking. One is that perhaps we should be aerating the 2nd batch, but this seems unwise and unnecessary to me as there seems to be ample yeast and the yeast has moved into the beginnings of its anaerobic state already by the time we run in the 2nd batch. Another is that possibly we should be leaving another day between brews (we do this already to good effect with our lagers) as perhaps the yeast hasn't moved far enough into it's anaerobic phase and would benefit from more time before the 2nd brew gets run in. I am thinking the 2nd of these is more likely. The paranoid part of me thinks it might be something to do with White Labs new (and totally annoying) new "milk bag" format, as this problem occurred exactly as these came on the market. Anyone else experienced questionable yeast vitality with these packs? Anyways just wanted to throw this out and see what the consensus was on this issue. Cheers.

PS Fermentation temp 19°C

[mmussen]

The first thing that jumps out to me is that its happening when you're using first generation yeast.
I've had issues with prop batches of yeast in the past simply because the yeast isn't accustomed to beer yet. Its been grown up in a lab and isn't used to the new environment its in. Particularly with a low pitch rate, that may be the source of your problems right there.
In my personal experience it seems to take 2 - 3 repitches before the yeast really starts behaving 'normally.' If you're not having problems when using yeast reharvested from other brews I would think that its simply because the yeast is from a prop.
In your set up do you need to use fresh yeast on your IPA or can you start your yeast elsewhere?

[johnny17]

It is certainly connected to being a first gen. However these problems don't seem to occur with our other first gen's even those using the same strain. It also doesn't occur when brewing this beer with only the single 15BBL tanks, even though the cell count is roughly the same for the first brew. This is what leads me to think it is connected to the timing of the second brew. It also doesn't occur when we brew 30BBL of our Belgian single that is 11.5°P. This beer has definitely progressed further into its anaerobic phase than the 15°P IPA by the time the 2nd 15BBLs is added, and there are no problems. Again, it seems to point to timing to me. Ideally, we pitch from a previous batch for our IPA, but we are still a small brewery and our IPA is very popular, so this isn't always possible.