What methodology is being used to poll SQL instances when using sqlcmd with the -L switch?
SQL Server discovery is attempted using UDP port 1434.

You'd need to get UDP traffic destined for 255.255.255.255 on port 1434 opened in your local router to see servers from other subnets.  

See http://www.mssqltips.com/sqlservertip/2057/why-listing-all-of-your-sql-servers-does-not-always-work/ for more info.