Yellowing Fan Leaves from Bottom Up

my info...
-soil/coco
-1000hps
-temp 65-71degrees
-soil a soil b nutes
good air flow veg state does great but about three or four weeks into flower im getting yellowing from the bottom up and plants seem to turn a light green color and don't fill all the way out. research is telling me that i need more n,p,k, and mg...and i noticed thats what the b side of soil a and B is....should i give more of the b or am i behind on my nutes? i water once a week then nute once a week. so they get nutes once every two weeks or about every 10 days.......any help please im in the fourth week of flower

Pictures help a lot, so post those up if you can. Yellowing leaves are normal as you get further into the flowering stage. Your plant is starting to focus it's energy on producing the resin in your buds rather than keeping the fan leaves alive. Can't say too much else without pictures, and I am basing my info. on my one time grow and researching around the site, so don't think this could be the only reason for the yellowing.

Add some pictures if you can and I'm sure some of the more experienced guys can help you out more than I did if it something else. Good luck on the rest of your grow man.