UDP problems

I am using RCM3710 and having problems with networking. I have a design that has been working well on its own network but now I need to run it on a general network with other data. I originally set the Remote IP to 255.255.255.255 but if the PC I need the module to talk to does not get a message in before others the module never replies to its queries. Tried changing the Remote IP to 192.168.5.1 which is the ip of my PC but after receiving the packet it has an error when trying to send. the return value of udp_send is < 0. retval = udp_send(&sock, buf, length);

Tried 0 but this is the same as 255.255.255.255. Tried -1 but I get an error when trying to send as when using 192.168.5.1

Here is my call.
if(!udp_open(&sock, local_port, resolve(REMOTE_IP), remote_port, NULL))
local_port=2003, REMOTE_IP is 0 or 192.168.5.1 or 255.255.255.255 and remote_port is 2003.

What could be the problem

Thanks
Trevor