I had the exact same issue and this is the article I followed and was able to use to resole it.
There are 2 documents you'll want to follow:
This one shows how to do the Bless process pre-netboot
In addition to the above article you should also watch the video in this one. Take note that there is one piece of the process in the video that was not documented on paper.
Step 2 is the piece that is missing the bless piece that needs to be done during the system image utility portion (you also need to do it to the individual Mac's prior to netbooting)
Both the image and the Mac being Netbooted both need to have the Bless done for each PXE Rep that will be deploying the Netboot image.
Since my organization has been using Deploy Studio, and it has been working I put this on hold to work on other projects. I was not blessing the image, so that may be the problem. I am going to try this as soon as I get the chance.