What is the pH of your water after adding nutes?
From what I've read, if you're having uptake problems it's likely due to the pH of your water. Our tap water can get up to 8.1, that's the highest I've measured. I keep my range between 5.9 and 5.5.
Also, sativa dominant plants seem to get nitrogen claw when indicas are just fat dumb and happy. I've increased the dilution of my nute/water combo for my satdoms, and the claws slowly began to unclench.
Different strains have different requirements, so what works for these makes those look like crap. IOW, it is not a "one size fits all" situation.
Check your pH, and if your problem plant is a sativa, dilute its food to a weaker solution than for the indicas. Also I have found that the satdoms can develop a tight root tangle at the base of the stem that will start causing water to puddle up instead of soaking through right away. If you notice this happening, the next thing you'll get is a stinky sulphur smell, because this is the ideal environment for anaerobic bacteria infestation that destroys the roots and smells like ass. I avoid this by aerating the roots as needed a couple times a month. This allows oxygen to get to the roots, the bacteria can't survive exposure to oxygen.
I' am on my third gorw and this is what I have learned so far. The most recent proves that third time's the charm as far as last time everything that could go wrong, did go wrong.
Cheers!