Are you sure your batteries are getting fully charged? The resting voltage on my fully charged W.E. 36v battery packs is about 39.7 volts. This is after being off the charger for at least an hour.
If you' don't have a multimeter, I recommend gettin one. You can buy a cheap one like this and it will really help in diagnosing problems with your e-bike
W.E. sent out a batch of defective chargers earlier this summer. The symptom of the defect is a rapidly blinking and faint indicator light at the end of the charging cycle. If you leave these defective chargers attached to the battery after they start to blink, they will discharge your battery down to about 50% charge. If you unplug the charger when it starts to blink, you can get a nearly full charge. If you've got a blinky charger, see this forum for a possible fix,
or if it's not been more than 30 days, see your dealer for a replacement.
If you're running full throttle, not pedaling and have some hills in your route, you might only expect 5 or 6 miles. The hype about 20 mile range is just that. It might be possilbe with a 110 pound rider on a lightweight road bike with high pressure tires running at partial throttle setting. In the real world, you'll probably only get half that range at best with the SLA batteries and a BD36 motor.
I have both a BD-36 and my wife has a BL-36. I'm quite happy with my brushed motor for my relativel short 3 mile commute. It's much smoother and slightly faster than the BL-36. It does use a lot more juice from the battery pack though. When my wife and I ride together, I will be at 50% charge when she is still at 80%. I like to attribute that to the greater efficiency of her brushless motor rather than to the fact that she is in better shape than I am, although I imagine both factors are in play here.