This happened in a SCCM 2012 R2 SP1 CU3 environment:
When Deploying an OSD task sequence via PXE, at the PXE boot screen the client was stuck on PXE – TFTP Download: smsboot\x64\pxeboot.n12 ……….
First thing I checked was the smspxe.log on the distribution point. I could see it was in a loop with the line “Looking for bootImage XXX00AA1”
In the ConfigMgr console, I checked Monitoring\Distribution Status\Content Status and verified that the boot image was successfully distributed to the distribution point. It was, but obviously there is an issue.
I went to Administration\Overview\Distribution Points and selected the distribution point having the issue, clicked on Content tab, typed in the boot image name or package ID and clicked Redistribute. Once the boot image had redistributed successfully, I cleared the PXE flag and PXE booted the client again.
It was able to successfully boot and run the Task Sequence.