Slow bud growth developement - AK47 in coco

Hello all

Hoping to get some advice about a prob with my grow. My key stats are:
-150W HPS sodium light
-Growing in coco
-Feeding with CANNA brand nutrients (Coco A+B (3mL/L, Cannazym 2.5mL/L and Cannaboost 3mL/L)
-Top down watering every 4 days, just switched to every 2 days - Started giving them 0.8mL/L of PK13/14 during the last week.
-10L pots
-2 plants in about 2,5 square foot cabinet (so around 6000 lumens/ft2)
-Serious seeds AK47, feminised seeds
-Temp during day 24C, temp during night, gets down to about 15C
- Veg growth for 40 days

My problem - bud development seems really slow! The pics below were taken at about 4.5 weeks into flowering (4.5 weeks after going 12-12). Going on other people's journals, they should be a lot further ahead by now. As per the photos, all I have is chunks of bud at each node, but not big buds covering the whole upper part of the stem like I should expect.... right?

I cant see any other problem with the plants, as the pics show they are robust and healthy. Heaps of crystals appearing. Very little if any dying leaves, etc, veg growth went off without a hitch... just a few tips but I think that is normal. They look potent... but just small!

Maybe AK47 from Serious slow? Could I be overcrowded? Lack of nutes - the PK I am now giving them should help? I havent seen any nute burn at all

Any helps would be greatly

there are a few things that slow growth that i have learned over the past few years

1) lack of light....250 is kinda small
3) bugs any soil bugs will destroy yeilds. cut a potato in half, place on soil. check with scope in 12 hours. any bugs appear on potato.
3) Ph. keep your ph around 6. too high will seriously diminish bud size. i just messed up an entire run of 12 plants by putting too much lime in my coco.

when in doubt, flush. if you have not flushed at 4wks you should do so now, then feed shortly after. you could be building up unwanted salt.

Hi thanks for responding. My thoughts:

- I think the light is fine. Sure, it is small (150W not even 250W), but the key is that the grow area is really small, only 2.5 square foot. Thus I am getting around 6000lumens/ft2, which my research indicates is OK. You guys might be using bigger lights, but I bet you have them in a lot larger grow areas.

-I did a flush last night, and will feed again tonight. Thanks a lot for this advice.

- I will try this potato test too, but I dont think it is bugs.

- Re the pH, I dont have a good meter, I just use litmus papers, which are not so accurate. But pH I think is around 6.0, and not over 6.5. I shouldnt have skimped on the pH meter... I will buy one for my next crop.

Thanks again for your help - if anyone else has ideas, please jump in

Willy