What would limit the number of simultaneous VPN tunnels?

A Draytek Vigor is only allowing a maximum number of 8 VPN tunnels. There are 17 tunnels configured, and when a tunnel is dropped another (seemingly random) tunnel automatically replaced the one that was dropped. No more than 8 tunnels are being created however, as though something is limiting the maximum number of tunnels that can be created simultaneously. What could be causing this?|||Probably a license issue. Check with the vendor about upgrading the license to permit more than 8 tunnels.