I have seen this on various occasions and most likely it is a licensing issue on the DS console. I usually do as a workaround the following:
1. Stop the Deployment Server services (PXE is not relevant to this);
2. Run the licensing utility against the Express.exe;
3. Start the Deployment Server services again.
Also check all computers on the DS console if they run a trial license or have an expired license. I found that in case you have 2+ clients in such a state, you will see such behavior.
-BBC
PS: Without knowing what version you run, I assume it is SP4 or higher ?