First you should check that the HumBug is configured to remove noise of the appropriate frequency. You can check this as it should be indicated next to the power connection socket on the back of the unit. It may be possible that the noise you are experiencing is not picked up from the mains supply. If the peak to peak noise is too large for the HumBug to completely cancel. If so, the indicator will continue to flash red. The HumBug cannot eliminate noise greater that 1V peak to peak. Likewise, low amplitude noise (<15mv) can be difficult to remove, so it is suggested that you increase the gain of your signal to ensure that the amplitude is greater than 15mV peak to peak.