I have implemented this algorithm in MATLAB and when I produce plots I notice that using Euclidean distance, I usually get presented with a clear pattern (sum of squares decreases with the number of iterations). However, when I run the algorithm using the modified Pearson correlation distance (1 - r, where r is the Pearson correlation coefficient), sometimes I would see no trend at all. In fact, on some occasions the sum of squares seems to increase with the number of iterations.
Is there a reason for this? Also I know that Euclidean distance is the preferred metric for this algorithm, but does it have any drawbacks?