What does your wattmeter show for wh/mile, or at least watts while riding, and at what speed? If you don't have one I'd recommend getting one, as it makes first steps of determining problems easier. A spedometer will also help, as just knowing the watts doesn't say how fast they're being used up. Knowing the terrain you're riding on and how you ride is also important.
If you're going fast and/or have hills and/or lots of stops/starts, your range might be perfectly normal. (but the motor would probably be getting pretty warm).
Next, your pack might not be balanced, and thus not fully charged, so leave it on the charger for a few days or weeks and see if it improves. If it's impractical to not use it for that long, then leave it on the charger for every moment that you are not actually using it, at home and at any destinations you travel to.
If you prefer to test/troubleshoot the pack, there are dozens of threads about how to do that in various ways.
The prolem could also be a number of other things, including the wiring order of phases and halls, but you will need to start the testing somewhere; I'd start with the first two above.