I'm using the Count points in polygon tool to estimate points and make some time-series in QGIS 2.18.4. I edit shapefile of points (just delete rest) to make a certain date range. As example: 1) 1995-2015 But in order to see the difference I must copy (Right click on layer - Styles - Copy) the styles from 1) to
Looks like a categorized renderer is used on the first layer. Most certainly, the second does not contain the exact same category values. That's why you don't see anything. Use a graduated renderer instead!