EAGAIN with write on a non blocking socket

when i write on a (DGRAM) socket i get often

EAGAIN
Non-blocking I/O has been selected using O_NONBLOCK and the write would block.

why? The buffer is about 105k and i think I am writing at a low rate very little packets
It is a control port with basical control commands

The problem could be there is another lower priority thread which is grabbing images (Gbit/sec)
May be the TCP IP stack is blocked by this thread despite it being lower priority??

Thank you