If they're in veg you have too much light.
40000 lux equates to around 750 umol/s/m2.
I use 800 - 1000 umol/s/m2 for flowering, some use 700-800 umols/s/m2 with great results
You only need half that for vegging as the lights are on double the amount of time.
Hope that helps
Yeah, I'd have to agree with Moflo here. Many have the missconception after yrs of using T-5 That 400-485w is ok to veg with & just raise accordingly. With T-5 you dont even have to raise at all. Set it till tje self topping starts.
With High efficiency LED 400-500 w is your flowering wattage over a 4x4+.
For instance, i use my HLG Trinity board (insane light/watt) at the lowest it will go, around 210w to veg & that's still 30" high on teens. Wouldn't even use it for seedlings or clones unless i had an 7x7 of them & 4' to raise it the fixture.
Now that's assuming you are Magging up already?. If not then unfortunate for many is this problem continues till losses set in & the rope looks enticing.
Lucky for you there's all kinds of new info. Regarding the prevention of LedDefficiency which in turn leads to a garden that will never let you look at hps again.
Soil growers seam to have this problem less. Of course this is only pertaining to your grow if you have the rest dialed in. Assuming you had successful grows in the past.
Good luck!
Vdp, Mag, cal-mag dont overdue it on Base nutes, less light (read plants) 1/2 wattage of hps to start. 36" high to start. They are young.