Hope someone can help, this all seems odd to me, I have a network that looks like this:

          bedroom media pc    office machine
                   |                               |
                   |                               |
                    --------------------------
                                    |
                         office gigabit switch 
                                    |
                         downstairs switch

                                    |
      ----------------------------------------------------
      |                             |                                |
lounge media pc    main file server             adsl router

Now I've been having trouble playing media in the bedroom media pc from the lounge machine (record tv from the lounge, you see)

So this morning I tried copying the file from the lounge machine to the bedroom, and also the office machine, and both copies were either very slow or failed completely, even though I can see all the files on the lounge machine.

In contrast, I can copy files very quickly to the main file server, as you can see it's attached to the same switch.

So is there anything I can check? I've restarted the office switch, with no effect and rebooted all machines. 

All machines on the network report gigabit connection speed.

Any help would be appreciated, I guess the only other option is to run cables from office and bedroom directly to the downstairs switch, thereby eliminating the need for the upstairs one, but I'd prefer not to have to do that.
Cheers,