gday mate
if your PH is correct (it's worth checking the soil) then it could be K deficient (leaves will eventually turn black)
also could be mites, check under leaves for eggs
or could be over-watering/or too soggy a mix.
If so, the condition will worsen and like K def. the leaves turn brown/ black from the bottom of the plant and the plant eventually die.
Check this by drying out the mix for a few days if it's not already and removing the pot (carefully)
If it is over-watering then the roots will still be a bit soggy, buried, grey and half dead, not white and running around the outside of the mix.
If O/W, gently knock off the soggy soil and replant in a better draining mix, with less coco if you're using it.
If it's fine and dry down there, feed it K. (The leaves look slightly Mg deficient too, so maybe it is nute deficient! get a better fert with all the trace elements and feed it seaweed as well)
hope this helps