Old merged druid segments still remaining in deep storage

With the druid.coordinator.merge.on=true, I could see that the data-sources are getting called for merge and the size of final merged druid segments increasing. But for some data segments, not all, the old segments which got merged still resides in the deep storage. What could be the cause of this scenario? I increased the kill.maxsegments but with no results. Let me know how to go about finding the root cause of why these segments are not getting deleted? The co-ordinator console shows the existence of only one segment, ie, the final merged file.