Why won't WiFi printer print from WiFi computer but will from LAN computer?

[#1]

I have two computers - a desktop (Windows8) and a laptop (Windows 7). Both work fine on LAN or wireless connection to my router for browsing, email etc. I recently bought an Epson WiFi printer which will only print if the computer making the print request is LAN connected even though the printer is WiFi.
I cannot ping the printer using WiFi but can ping it from both computers when they are LAN connected.  I CAN ping each computer from the other when on WiFi so AP Isolation is not the problem.
I can't see how it is security software (ESET on desktop and AVG on laptop) because both can print when LAN connected. I can't see it's encryption problems (WPA-PSK (AES) because both computers work fine WiFi connected to the web etc. and the printer does print via its WiFi connection if a requesting computer is LAN connected. My router sees the printer and is obviously happy to send it print jobs from a LAN computer.
Where is the problem? It's very frustrating! 
Thanks for any assistance,  Mike.

[#2]

Have you tried it without the ethernet connection?

[#3]

Can you ping between computers if one is LAN (Ethernet) and the other is WLAN (Wi-Fi)?
What are the IP addresses of the computers and the printer (both wired and wireless for the computers please) and the subnet masks?

[#4]

what router are you using? Some router have a option for ISO (Wireless Isolation) witch  isolates the wireless network from lan.
Also the TG582n (PN router) has a "Network File Server (Windows Networking)" for setting up the windows sharing. I'm not sure if this will help.
Plus did you check if the wireless adaptor properties and make sure you have "File and printer for Microsoft Networks" and "Client for Microsoft Networks" installed and enabled.
This might be a stupid question but can you see your other computers connected such as the host computer with the printer connected to or is the printer connected via Ethernet? Can you add the printer from "Devices and printer" and "add a printer" then "Add a network printer". If you printer is connected to a computer thats on your network then you need to join the "workgroup" or what ever you called it, to be able to access the printer.

[#5]

Presumably you had a installation CD with the printer....
IF so... run the installation CD with only one of your computers "live" and only WIFI connected to the router...
this should set up your printer in the WIFI mode....
You could then run the installation CD with the other computer "live" and see if it then sets up the printer for that  setup....

[#6]

Firewall problem? I mean Windows firewall (or some other software firewall) on each computer. Perhaps Windows firewall is blocking more things because it doesn't trust the Wifi connection as much as the LAN connection.

[#7]

I have never ever seen the Windows firewall giving problems.  Other firewalls that come as part of some or other security software, yes.  Windows, no.