One common method a website can use for blocking VPNs is to simply identify the IP addresses employed by various commonly-used VPNs and block them. Other methods involve coding their websites in ways that the VPN can't directly (or correctly) respond to, and block them as a result.

While some VPNs may be better able to bypass a given website's "VPN sniffing" techniques, it's hard to predict specific results without knowing details of exactly how a particular VPN operates or a target website does its sniffing. The tendency seems to be that the simplest VPNs are more susceptible to being rejected by a website, but in any case, a website (or even a national censorship "firewall") that is sufficiently determined will always find ways to block a VPN. All you can really do is try some other VPNs and see what happens... or else find another website that offers similar content without blocking.