My first suggestion would be to check your PH.
Phosphorus can be locked out when the PH is too low, and Boron can be locked out when the PH is too low and too high. That's what it looks like to me but I'm not an expert. Here's an excelent link with pictures to use as a future reference.
http://www.icmag.com/ic/showthread.php?t=11688