I'm following the online qgis manual available on qgis website. I'm doing the step 21.11.3. (http://docs.qgis.org/2.14/en/docs/training_manual/answers/answers.html#complete-analysis-3).
To make it simple : I have a raster converted to vector, so now it is a polygon (several polygons). I'm doing an interior buffer exactly how it is described in the manual. I didn't obtain the same result. I can see the interior buffer just in some polygons, not in all of them ->
The interior buffers appear just for those polygons, that don't contain a hole. I use qgis 2.18.3.
This is how i created the interior buffer ->
In green you see the input layer : suitable_terrain, in blue the output layer, the interior buffer.
It is not a problem of a projection. I use WGS84 UTM Zone 34S where the units are in meters. I did also the Check validity and i obtain the same result.