Just remember what you put in now will go up in smoke later. If the problem is not real bad (insert pics here) you could just PH as squid stated. Most likely the nutes are in the soil already just locked due to 6.3 PH.
I have my soil like you at 6.3 we are both going through the same exact issue Mandy. I upped my PH for my waterings and back down for the feedings. 2 of the 4 plants are showing CA,MG currently not real bad but enough to concern me as I am way behind you on my grow.
My G-13 has caught up to my oldest Blue Mystic and it's not even a month old yet. Damn thing is flowering profusely and I'm not giving it anything to induce it, other then the soil. It is showing signs of CA,MG at 3 1/2 weeks old, crazy aye.All in the PH
http://youtu.be/jNzlzDn6Ioc
Check this out at 12,500 feet my outdoor test plants. Got a little tribute to some of you in there..