I don't mean to hijack your thread since I've already posted by sob stories elsewhere on HBT but I fermented and dryhopped in my serving keg which prior to any temperature drop, I pressurized with CO2. It seems the cold crash took all the hops and aroma with it. I'm using a floating diptube, so it was taking very clear, light beer off the top. I gave the keg a little shake in hopes it would redistribute the hop flavor/aroma, which it did to some extent, but it seems I'm the odd person out here, since most people separate the beer completely from the hops with their transfer and I've never heard anyone say the physical separation pulled away the flavor/aroma. Last time cleaned out my keg hopped beer after it kicked, the aroma in the remaining hop sludge was overwhelming and pleasant. Yet I can't seem to get that to infuse into the beer.
I had similar problems like you when I was dry-hopping in my 11liter fermenter kegs after a cold crash, before I transfer them to my serving kegs. I didn't seem to be able to extract enough from the hops. I read up a bit here on the board and it seems it could be a combination of lots of things: the pellets just dropping straight to the bottom (pellets being too cold when dropped in, and my kegs being shorter than 19l kegs and resulting in a shorter travel time from top to bottom), still too much yeast present in the fermentation keg dragging down hop oils, and the inability to rouse the hops without bringing the yeast back in suspension.
So I switched to the following steps:
* purging my 9.5liter serving kegs (that are filled to the brim with star san) with fermentation CO2 from my 11 liter fermentation kegs
* no dry hops before terminal and cold crash
* cold crashing my fermentation kegs to 0-2C for 2 days to get as much yeast to drop out
* taking my hops out out the freezer plenty of time before dry hopping and also squashing the package a bit, so the pellets already break up a bit
* using a mix of T90 and Cryo (1:1) for dry hop to reduce the amount of plant matter in my serving kegs
* dropping those pellets in the purged serving kegs while flowing CO2 into the serving keg
* closed transferring my beer from the fermentation keg (which has a CBDS) to the serving keg (which has a CBDS) by using gravity
* dry hopping for 3 days at room temperature and 20-30psi in the serving kegs (the pressure helps to lock in the aroma in the beer), while flipping the kegs twice a day, in order to let the hop particles "travel" a sufficient long road through the beer. (see comment later about professional brewers and residence time of hops)
* finally cold crashing the serving kegs
I've only done this once, but I now have a 5.5% Citra pale ale ( 4oz of Citra T90 in the whirlpool, 3.5oz of Citra T90 and 3.5 oz of Citra cryo in the dry hop) for a 5 gal batch on tap which is bursting with flavour and aroma and which has a defined citra hop character and a stable haze.
It is definitely the best hoppy beer I have ever brewed and I hope I will not get any vegetal flavours later on from the hops still being in the keg.
I think the key is dropping out as much yeast out as possible and moving the beer to a purged vessel so you can sufficiently agitate the beer by gently flipping the keg twice a day. Professional brewers have less this problem of extraction as the hops have to travel a longer time when they are dropped in at the top of the fermenter and travel to the bottom of the fermenter and even they still rouse the hops with CO2 to get full extraction. In the end extraction is all about the residence time of the hops in the beer and the environment they see while travelling through the beer (temperature and hop oil concentrations). The flipping also helps with the mixing of the beer, so the hops see less high concentrations of hop oils while travelling through the beer so there will be more extraction for that specific hop particle at that specific time.
The only problems I see in this approach is leaving the beers on the hops for the whole time they are in the serving keg, but if you think that the problem in your and mine old approach was that the hops were sitting at the bottom not getting enough extraction, I'm thinking that in this case it is a good thing that the hops are sitting at the bottom of my serving keg with only slow extraction any kind of vegetal flavours at the slow rate while the beer is taken from the top of the keg with the CBDS.
Another problem I could have is possible diacetyl because of hop creep. Solutions for this could be adding a small amount of pellets during fermentation so the creep takes place when sufficient yeast is still in suspension resulting in no diacetyl or dry hopping after terminal at lower temperatures (< 16C)
Big shout out to everyone on this board and especially the regular contributors to the Northeast Style IPA thread for helping me find a way to make bright hoppy beers which are on par with a lot of the NEIPAs that are now in the market. My specific method is just an adaptation of what other people on this board have been suggesting for a while now to my specific situation where I work with shorter 11l kegs to fit in my fridge.