Mg deficiency takes over 30 days to see the symptoms from onset of the problem, so if they aren't that old, it can't be Mg-def.
not enough info.
irrigation habits -- always irrigate with fertilizer at 1.3-1.5 EC and religiously get 10% runoff waste or more -- toss it down drain or outside, do not let the plant sit in its own piss. Repeat when top layer starts to change colors. Can wait a bit longer, can go a bit sooner, early on i'd suggest more of a wet-dry cycle as that will promote deeper roots. can ramp frequency up in bloom if you want, as long as at least 33% of the pot's weight is lost before re-irrigating. (weight loss = water use)
this looks like something related to bad watering habits. the odd leaves are probably somethign to do with ratio of individual nutes or genetics. usually a happy plant will progress from 3-5-7-9-.. fingered leaves each new growth node it adds. when it doesn't that's a potential sign feed is off a bit. not all plants will give you 9-11-13+ fingers, but you should get to 7 or 9 easily with just about any plant.
start tracking your nutes.. either weighted average of the % off the labels as they are mixed or calculate ppm of each. one or the other is very useful when adjusting formula, initially... once you have plants happy seed to harvest, you can relax about that stuff, but until then it will greatly reduce learning curve.