dhcp-relay does not seem to relay bootp requests

there’s a central dhcp server running in our network, and we have a routed net with a couple of subnets.
while normal DHCP relaying works pretty well, i can’t get PXE booted stuff running using mikrotik devices. we decided to
ease the os deployment process, so some PCs on a subnet connected to a routerboard would be netboot from time to time. this however does not
work.

i saw bootp support under “dhcp-server” but nothing similar under “dhcp-relay”.

theoretically it should work, i guess. but it does not.
any clues?