Issue with SCCM pxe deployment on dell laptop

Hi everyone, 

In the last two weeks we have a weird problem at sccm pxe  deployment, The deployment of the OS works for all types of laptop except Dell E6440 latitude (it worked before) . Now in those machine they give me PXE-E32: TFTP open Timeout message when i try to boot from pxe.

Any help will be great . 

Thanks