Question on SSDR NB

The addition of the WNB has really improved the working conditions at my QTH.  I get a lot of power line noise and the WNB does a pretty good job most of the time.  I also like the fact that it reduces the noise on the panadapter.  Sometimes I turn on the WNB and I still have some noise which the regular NB takes out.  The NB doesn't affect the panadapter but as least it seems to work pretty well taking out the buzz.  My issue is it seems to degrade over time.  I'll turn on the NB and run the slider to the lowest point which still takes out the noise.  If I start tuning around, the NB is still on but it stops working.  If I turn it off and back on, it starts working again but after a short period of time, it's no longer effective.  This doesn't just happen when tuning.  For example, if I am listing to a weak to moderate station, I'll turn on the NB if I have noise.  It will take out the noise but then if another stronger station comes on the same frequency, the NB looses it's affect.  The NB has been like this since before the WNB was introduced.  Does anyone else notice this?  If the NB would stay active, the combination of the WNB and NB at my location would really be nice.  I find myself constantly turning off and back on the NB to "reset" it. 

Burch