PXE Boot issues

We have recently had over 100 Precision 3660 workstations delivered to be installed into suites. When we deploy the PXE image to the suite some devices pick up the PXE without issue, others get to the contacting the server screen & begin to time out. We have discovered that when they are stuck on the contacting the server screen if we clear the PXE at this stage in SCCM then it kicks into life & downloads the build. 

Does anyone have any idea what might be causing this issue?

We have tested in older suites with existing devices & they download with WIM absolutely fine with no need for a 2nd PXE clear.