Q: DHCP problems, tried all the standard fixes

Hello, hope someone can help with this long-running issue, here’s our set-up:

UK BT broadband – Belkin Router – Belkin Powerline Adapter – Airport Extreme

The Airport Extreme is in a central location in the house in another room from the Belkin router, it is set-up in bridge mode with the Belkin Router managing DHCP.

In another room away from the Airport Extreme and opposite side of the house to where the router is, there is an Airport Express that will consistently connect to the network wirelessly and gets an IP assigned from the router. All looks good on Airport Utility on my Macbook (10.9.4, late 2008 model) when viewed wirelessly. However, the Mackbook in the same room as the Airport Express, does not receive a valid IP address. So I can only connect to the internet by physically connecting (ethernet cable) to the Airport Express.  I have tried on numerous occassions rebooting everything working from the Belkin router to the Extreme to the Express, but this does not resolve the issue. The Airport Extreme and Express have no problem connecting to the network every time, and they receive a valid IP address from the Belkin router.

How can the Airport Express connect wirelessly via the Extreme and get a valid IP address from the router but the Macbook in the same room with the same settings can't, do they have very different wifi adapters?

Thanks for any help you can offer.