This has gotten to be a major problem at our company and depending on who I ask, the problem either does not really exist (mfr. and vendor) or is insoluble ( according to most users including techs who know how to prove their point).
The problem involves getting a normal Windows 7 system to connect to a normal Server 2008 R2 Server over a cellular router (usually called a Mifi). A very few brands/models appear to work but the majority cannot make the connection. Since it is a cellular device, there are many variables that come into play and I wondered if anyone had ever found a consistent way to either make one work or else prove to the providers that their equipment was at fault. They all specifically state “VPN use” on the sales brochures. But few if any work. And those that do are not reliable.  

From a standpoint of pure knowledge, I just wondered if anyone knew the real reason why they fail?
Pptp, L2tp, IPsec doesn’t matter. I have not tried Shrew or OpenVPN and am using strictly MS Windows protocols. Plenty of Google Searches back up my complaints but none seem to be any closer to knowing "why" they fail, just that they do.  

This is a "quest for knowledge"question. I don't expect a solution. Just a reason for the problem if anyone has any ideas.