Attach your VI !!!! Then we can see how you have it configured.
That subVI is polymorphic. If the instance handles one channel, then it outputs a scalar. If the instance handles N channels, then it outputs an array, each element is a value calculated for each waveform. If you only put in one waveform (a 1-D array of waveforms with only one element), you'll get a 1-D array of amplitudes with only one element.