yellow tips

can anyone tell me why i might have yellow tips on my plants

Pls post pics there are many reasons but without pics it makes it very hard to even guess.
Welcome to RIU

yellow tips huh, mmm, that's not good.

I betting you burnt your plants~ Do you have brown on the edges of the leaves?

A pic would be helpful!

hi guys, ty for the welcome and the quick reply...i will try to get some pics but for right now i;ll try to xplain w/ more dets....all bottom and middle leaves are a beautiful lush green it was just towards the top the very tips of the leaves have yellow. i had my light about 16" from her and have recently moved it to 24". her buddy next to her doesn't seem to have any yellow tips and she is treated the exact same, so i just didn't know????
also, any advice on swithing to flowering? (1st timer)....i want to switch today, will it harm the plants to stop in the middle of my 18/6 (i want to have everything ready to go by morning...ie 12/12) will i disrupt anything by cutting todays light period short by 14 hrs? this would have my girls in darkness for 20hrs. thanks!!!