Unless you are running 3B+ pis, the firmware code for pxe booting has several bugs. Also, Pis have issues with retransmission of DHCP requests on low end network hardware that doesn't set link up fast enough to hear the Pis intial dhcp request. Almost all of these bugs are fixed one way or another at this point though, the github issue tracker for raspberrypi/piserver and friends has lots of info.