Assuming that the effect is being applied to both the audio channels of the clip, and that "bypass" isn't activated, the remaining hum could either be because of a harmonic of the 60hz hum, or that the hum you are hearing isn't mains (60hz) in nature.
The best way I find is to set a single filter to "notch", give it the widest possible width and then whilst playing the clip slide the frequency up and down. You'll hear the affect of the filter in real time, so you should be able to zero in on the offending frequency.
Once you've found it you can then narrow the notch to filter out the hum but leave other nearby frequencies (that might be part of say speech) less affected.
You may need to use another notch to catch a harmonic if one is present.