Stretching with very thin leaves and slow new growth

Lemon OG in 22 litre biobiz soil, 4 weeks veg

This has been doing very well but suddenly all the new growth slowed right down, branches stretching and new leaves very spindly. There obviously some nute deficiency in the new leaves too which are darker at the veins. Anyone have any ideas of the cause? I have Tangie growing next to it in exactly the same conditions/age with no problems.
Ive looked at Zinc or Iron but don't think thats the problem.

Some signs of heat stress, and those top thin leaves can be a sign of broad mite damage. Interveinal discoloration at the top would generally be immobile nutrient issue, not sure what specifically. Good luck!