There are many vpn solutions, so there are no general answer to your question. E.g. Spark works fine for us using Fortinet Fortigate VPN solution. So it must be something with your VPN configuration (blocking something maybe) or other network issue which is very hard to investigate. At least provide the name of your VPN hardware/software, so maybe someone using same stuff will advice something. Though, most probably you won’t receive any reply on such topic.