So, basically if you can make sure that when you try to ping the default gateway from the client , the client does have a valid IP address , there may be following issues:
>> The client drivers
>> Client side firewall
>> Client going to sleep
However , I guess that all the other clients in the office also have the same drivers, same firewall setup and same power management settings configured via Group policies. If not , then checking the points mentioned above may help. The next step would be an in-depth investigation with wireshark captures.