Lasagna Bed Update - Feast and Famine

Massive!

Front of the bed struggles, while the
rear of the bed flourishes. 

What is going on in my garden these days?  Well, it is pretty much feast or famine.  Certain things are growing like crazy (corn, squash), while others languish (peppers, certain tomatoes, onions).  I don't quite understand why this is happening, but I have a few suspects (more after the jump):