Great thread, I've been going through something very similar and have been troubleshooting using the exact same steps as Newty... Week 5-6 flower gets me every time. My grows are slowly improving, but I see the exact same symptoms --
All of the reading I've done points to:
Soil PH (this is the number one recommendation by many)---Tested all of my grows runoff at around 6.5
Lockout- I've tested PPM runoff PPM's, etc... All looks good..
Underfeeding/underfeeding- Tried just about everything here, more less, etc... Still happens...
Pests- Dammit I may have them, but you have no idea how many hours I've spent searching leaves, roots etc...No pests I can see...
Fungus/ disease- Again, hard to say... I'd have to send leaves out to test..
Temperature/ Humidity--- My temps are a little low, but I don't think this is it (72-74 degrees) Humidity around 60 veg and 50 for flower...
Light distance--- Tried just about everything !
Cal/mag--- Used this and it did help, but I still think something is wrong..
Soil: tried many ! Ocean Forest,
Happy Frog, Coco, and Coco Loco.... Same problem with all...
After all of this, I'm still unsure--- So, I have a few more tricks to troubleshoot with..
Allowing this soil to dry too much in between waterings (I do this)--- Someone else mentioned this in your thread....
AirFlow--- My cabinet has a 4 inch intake (passive, I may try active)... I may also add another 4 inch intake if I stay, passive.
Temperature-- I'll try to bump up to mid 70's next grow.... (lights off can go 68 for me)
Roots overcrowding--- Doesn't make sense as I've used 3 5 and 7 gallon pots (cloth or the plastic ones with holes)
Roots are slighty tan brown at harvest and very thin..... I'm not sure if its a root problem, but it could be. No mush, no smell, but not wilky white like my roots are when they are younger...
Less time in Veg--- I may try to lessen my Veg time as I like to let them stay in veg a lot longer than most..
Try a new strain--- Maybe its just the strains I'm growing
Anyway, you have no idea how helpful this thread is.... I'm taking notes ! Sorry to type so much, but this has been driving me insane ! Its like looking for your car keys in all the spots they should be and there is no place left to look !
Side note: My plants look amazing all the way through Veg and early flower, picture perfect... It's just mid flower that gives me trouble.. I get some very good buds in the end, but I'm sure they could be better... (I'm sure yours will still give you some tasty buds as well.) Kudos for not giving up !