Is there an exemption list on the wireless?
I suspect that the wireless users are placed into one VLAN, with a unique subnet, and the Desktops are in another VLAN, with another unique subnet. The issue then is most likely that the LAN firewall is configured to allow DropBox access from Desktop's subnet but not the wireless user's subnet. I commonly see LAN firewalls configured so wireless clients can only access DNS, DHCP, HTTP and HTTPS. This type of firewall configuration will stop a large number of Internet based services, such as DropBox.