This can happen if the ip addressing at their home network conflicts with either the ip addressing behind the vpn, or the one used by the vpn. I had this happen with a few clients, and didn't realize what was happening until determining what their home network was set to.
In my case, the work network used the 192.168.2.0/24 subnet, which was the same as my client's home network. so he could connect just fine, but when trying to reach the company server 192.168.2.2, nothing would happen. The reason was that according to his routing tables, 192.168.2.0/24 is a local subnet. So even setting the VPN server as the default gateway didn't work in this case. The solution was to set a route that would send all packets destined to 192.168.2.2 directly to the VPN server.