one sickly looking plant out of all of them

Attached are a couple of pics of my grow tent with a sickly looking plant at the bottom left.All of the others seem to be ok but this ones leaves are starting to droop and feel slightly rubbery.Temps ok,light's ok,soil's moist not wet,ideal conditions.Any ideas anyone???

maybe it caught a cold? poor little girl

weird, looks over watered but you said the soil isnt too wet. have you been spraying the plants at all. maybe shes taking longer to dry off or someting. looks like shes the furthest away from the fan. seeds or clones?

All from seed,it's nice and warm in that corner,it's not overwatered,haven't watered any of them in four days cos the soils still showing moist on the meter besides i had some overwatering problems to start with so i aint going down that road again,stunted their growth for a couple of weeks.Mmmmm,can't work it out,it's the only one out of twenty that aint buzzin'.Oh well,19 out of 20 on my first grow aint bad I suppose.Thanks for your comments
Oh,Hotfuzz,when you said have I been spraying them,is that bad or good?I've had problems with low humidity at 20% so for the last three days i've been spraying the top of the soil with just water and around the tent to get the moisture up.That won't harm them will it?They seemed to love it.

spraying is good but maybe try spraying less on the wilted one. since its seed it could be something genetic