I would guess - and it's genuinely that - at one of the following, in no particular order:
1. You don't have access to the tuners (video group membership or similar) - what user:group are you running as?
2. The tuner hasn't initialised correctly (firmware or driver) - what's in dmesg/syslog?
3. Could be a bug in tvh - does wscan/scan_s2/whatever have any better luck, or generate a more useful error message?
As I said, guesses only, as I have neither a Pi2 nor a diseqc switch... actually...
4. You do have a switch, don't you, and you're not configured to switch what isn't actually there?