Okay, totally different issue then. Client roams to another AP and maintains an IP address (Assuming that you are somehow verifying that the client still has an IP address - Is it the SAME IP address?). And if the client roams back to original AP, the PING replies resume. Interesting.
So after the roam, client no longer responds to PING requests at a known IP address....or what you believe to be its IP address.
Log into the AP that the client roams to (and the client is no longer PING'able) and from the CLI run:
#show wireless client
Do you see the client listed?
You should...and you should see the device MAC and an IP address.
Is the IP address what you expect it to be?
If so, try running another PING against it while running the command on the CLI:
#service pktcap on bridge filter icmp
The AP should be proxy ARP'ing for the client. But, if the AP isn't showing the client and the WLAN's Proxy ARP mode is set to strict, then I guess that's a scenario when there might not be a reply.