MacBook Pro wifi works but airport utility sometimes shows warning triangles

Context

I have a MacBook Pro running OS X Yosemite as well as various other OSX and IOS devices.

I have an Airport Express (called groundfloor) running in bridge mode to a VDSL modem router and another airport express (called topfloor) that extends groundfloor.

Problem

Every IOS or OSX device in the house except the macbook pro sees the two airport expresses and shows two green lights when using the airport utility.

If I turn the macbook pro wifi off and on again it I can see the airport expresses with green lights when using the airport utility. Then, within 30 mins, the green lights will switch to yellow warning triangles on the macbook pro. Wifi still works on the macbook pro and I see the "internet" with a green light.

Please can someone help we work out why the green status changes to warning just on the macbook pro. Thanks to the new streamlined approach of the Yosemite airport utility I appear to have better logging on the windows version. Maybe there is some spy utility I can run to see what is going on?

I've posted this question under Airport Express as the Airport utility is the one reporting the problem. I appreciate the problem might be something specific to the Mac's connectivity settings etc.