what lighting are you using? if the light is too intense the plant will slowly lock out nitrogen so the leaves will yellow, then they start to burn ....i mistook this once for nutrient burn so stopped nutrients, flushed and it just got worse...i found this yesterday which might be useful .
I wonder if you have given it too much calcium ? calcium competes with nitrogen and i think potassium on the root space ,
www.cannabisbusinesstimes.com/columns/cultivation-matters/news/15750558/feel-the-cannabis-leaf-margin-burn

just found this from a spanish grow site.
Excess calcium is difficult to detect, as it causes the blockage of several nutrients, creating deficiencies in potassium, magnesium, manganese and iron. If this is your case, you will notice it because the tips of the leaves of our plant have small black dots or have been burned. The plant, having excess calcium and, therefore, preventing the nutrients it needs from reaching the plant, also has many difficulties breathing and the result of the effort to achieve it is precisely what we have just mentioned.

• The newest leaves wither
• Growth can be stunted
• Blocks the absorption of potassium, iron and manganese