Dlink DI-614+ router - connectivity issue

Hi all

I am using the above router to connect 3 PCs to the internet; the PCs
are all issued an IP address by the router's DHCP server, and they all
connect to the internet without a problem.

The problem is when trying to share resources between the 3 clients -
they can't see each other. Even though each machine has an IP and is
on the same subnet, they can't ping one another, yet they can all ping
the gateway.

Any ideas? Amy help is appreciated!

Dale