Watts always at zero when the road declines

[jsshort2302]

Here in N. Ga we have no true flat roads, you are either climbing or descending. Since I started using the PP, my watts go down to 0 when descending, no matter if I am on a long steep descent in a non pedaling tuck, or on a slight downhill and still pushing a big gear hard, zero watts are displayed until I start going up again. Normal or what? I never really got a good warm fuzzy about my calibration ride but it did go slowly to 100 while I rode and then started displaying a number that I assumed was it's estimated power output by me. Is there a way to make the unit go into a calibration mode at any time? Thanks.