@Carmen Ray it does look like an early-ish potassium problem. It also looks over watered, which I know it's not so that tells me that it isn't moving water properly. It kind of never really has.
If you have any liquid kelp or seaweed extract, that would help in the form of a liquid feed, but I think it's something else causing a K def and poor transpiration.
I'm leaning towards myco not being strong enough. Sometimes at uppot the new myco dusting just doesn't take.
Talbourne nourish comes in 2 different formulas. Which did you use, 7-1-2 or 5-1-4?
If you used 5-1-4 Then I would just hit it with a myco drench. The mixing instructions on your myco likely have a drench recipe.
If you used the 7-1-2 then I would use a feed of liquid kelp or liquid seaweed as per the mixing instructions and a myco drench.
Hopefully you used the 5-1-4 and you may see some relief quickly. Being that it was a foliar pay attention. It will have an effect quickly. 3 or 4 days, sometimes 2.
If it works you know it's a K thing and you can get a couple soil feeds in to get caught up, then work it in to your schedule as a feed, and when you rebuild the soil maybe you need a stronger K source.
It could just be that plant too, I've seen lots of auto grows where 1 just isn't happy and the others are just fine. They can be quirky.
I'm not personally a foliar fan, but I think some calmag and some Nourish should have a positive effect. I think it's a great response on your part. You should see results quickly.
To me it looks like either a cold issue, and you have a heat mat so thats not it, or a myco thing.
Hit it with a myco drench for sure. Apply it very slowly and stir it constantly. You don't want any runoff unless it's out the bottom, and you want it all dumped into the root mass, so closer to the stalk mostly. It will spread it's web, but in needs to contact roots 1st.
If you didn't need it then no harm can come of it, but if it's the problem then it will fix everything.