Can someone help me out in figuring out what wrong...

It's not absorbing nutrients as well. Maybe has root rottt starting or poor drainage . and can't absorb the nitrogen etc micro elements etc it needs

It appears to be 1 of 3 things: 1.) It's a different phonotype; 2.) The bright green coloration compared to the other suggests pH of soil is different than front plant, which is allowing back plant to absorb more nitrogen but not enough to burn roots bad; or 3.) The LST stunted this plant, which resulted in it being skinny. It could even be a a couple of the above issues combined. *Solution: Test pH of soil, 5.8 preferred. If below 5.6 or above 5.9, flush soil with 5.5 pH water with no nutes, wait 48 hours & use 10-15-15 ratio on fertilizer. For faster results, add 1/2 teaspoon of powder root hormone to 1 gallon of water to build root system to absorb more nutrients. Bigger root system = bigger plant faster. This will heal root burn in case that's part of the issue & will allow plant to grow faster & make up it's lose in size in no time. Hope this helps. It's is an auto, hurry. If it's a phenotype, you'll have more time.

Hungry for nitrogen, probably as a result of an over enthusiastic and unnecessary flush. However, some strains with lots of Sativa genetics, are naturally light green. Defoliation probably hasn't helped either.

It's a little pale... so not a N tox like you think in diary. a few things can cause burnt or rusty tips. Could be too much P. or, too little K... You have to consider what you have fed... familiarity with the soil you use etc... Check soil pH with a slurry or runoff... make sure that isn't drastically off from 6-7ish range. do this first before any adjustments. use a nutrient ppm calculator to see if you are blasting it with any particular molecule... With soil this is a bit more art than science as you have to guess what the soil already has.. but if what you are adding is out of balance, it'll eventually cause an issue.... or, maybe the soil comes out of balance etc... if the latter is true, it'll probably take a while to fix. Good thing the other plant is healthy? ... so it can't be far off, if giving similar diet. irrigation habits? are you ensure entire volume gets wet when you water? whether with fertilized water or plain water, you want the whol thing to get wet and that means some minimal runoff is neccessary. you don't need much with soil unles trying to correct the balance of what it contains in more extreme contexts than this.

Could it be a ph problem?

Hard to say without any info.. In that picture I see a plant that is a little bit light/pale (could just be genetics, and the lighting in the pic, as sativas are sometimes a bit lighter) I think on the left side I am seeing a couple leaves curling sideways possibly could be my eyes playing tricks on me. Also seeing some minor burnt tips in some places. How heavy (ppm) are you feeding and how often? What is your Ph of water/nutrient solution? How often are you feeding and watering? Are you letting soil dry up between waterings/feedings? What kind of light, how far is it from plant? And what are temp and humidity like in growing environment? It doesn't look like a N deficiency or it would be worse on older growth and fan leaves since N is mobile. I doubt its heat related, or it would likely have other symptoms as well. Doubt its light related as it would cause issues at the top more than bottom.. Could be ph or nutrient lockout or could be over watering (not looking droopy atm though) All of which could cause deficiencies, bent leaves and tips to look burnt. Or could be something else.. Hopefully someone chimes in and helps ya out.

hard to see it. But I'm not seeing much of anything other then its a bit lighter shade of green vs the other plant. Best to link your diary to the question. You will get better feedback from growers as the information they need is available. I think its a none issue at this point, but cant full see it so this is a shot in the dark. Good Luck!