Hi Lisa, A common problem is iron deficiency. It 'may' be due to your PH being a little high. I keep mine at 6.5 - 6.8. That's what works best for 'my' plants.

I add Maxi-crop 'with' iron. I buy mine at a local hydro store. It will darken your water if you add too much, too fast. Hey, it's a guy thing....'more' has got to be better, right ? !...

I also add azomite, which is loaded with different trace minerals. Works 'great' in the dirt garden...we have 'nothing' in our soil, aka 'sand' here in FL. My 'maters actually have some flavor.
In AP, I add it to the water, just before it goes into the gravel beds. I have sprinkled it onto the gravel and rinsed it in.
http://www.wheatgrasskits.com/azomite.htm

This might be worth saving to favorites....
Here's a link, to a picture guide, showing the results of lack of minerals.....(had it saved, don't remember who posted it first ? )
http://www.hbci.com/~wenonah/min-def/list.htm

Hope this helps...good luck Lisa