What could it be?

At this point, you can see how this gets tricky. Many growers not familiar with nutrient lockout may misdiagnose their plants. Because their plants appear to be starving they think that they need to feed them more nutrients. Of course, this only adds more salts/nutrients to the environment… which negatively effects the pH more… which causes even more nutrient lockout. So be sure not to overfeed your plants because you could make the problem worse at this point.

My guess as you have several different issues would be nutrientock out probably due to incorrect pH levels check the ph of your substrate if they are not correct flush water with correct ph'd water through let em dry and restart feeding keeping close eye on the ph levels

You can start a diary any time during a grow - and it would be good to do so here.... you've definitely got a deficiency but why this has happened will only come to light with some information! It could be a nitrogen toxicity that's locking out the calcium and magnesium, it could be a pH that's in a terrible place and the plant can't get at the nutes it needs... but we need a diary. Just put together as much of one as you can and ask your question again! And btw, plants are individuals... even though you may feed them the same, there's no guarantee they'll react the same... it's kind of like expecting twins to be perfectly identical.... they're not.