Diagrams not drawing on QGIS

I'm trying to draw a pie chart on QGIS, over a polygon layer. This pie chart should include data from 6 fields in the attribute table. These fields contain the percentage of several categories of data and all of them sum up to 100% (I've checked it personally). However, the diagrams option in layer properties gives no result. Anyone knows what may be going on?

Try turning off simplification for the layer, under the rendering tab. That might be causing invalid geometries which prevent the diagrams from being placed correctly.