In reply to a post by Andrue:

One thing I will say is that both the printer and the desktop spend most of their time powered down. Although why that should cause an issue on the LAN I don't know.

Does your router turn off unused ethernet ports to save energy? Have you tried disabling the feature, to see if it makes any difference? In my router the option is located at:

Advanced Setup / Management / Power Management / Ethernet Auto Power Down and Sleep'

Interesting that it started working after pinging as that does suggest the routing tables are not updating properly.

I was going to post about AP Isolation that could cause the issue you were seeing. By turning it on it would stop wireless devices from talking to anything other than the Internet. It seems that probably isn't the issue but worth checking anyway just in case. You can find it on page 29 of the manual.

AP Isolation would almost fit the bill except that I could see the router. Definitely seems like the WAP is at the heart of this and - somehow - it doesn't see new devices appearing on the LAN until 'forced' to go and look for them.

I believe IP isolation would still see the router as the device would know it as the gateway and so allow traffic to it. However, it does sound more like routing issues either in the WAP or on the device.