I use QGIS 2.14.11-Essen and PostGIS 2.1.8. This is an issue I had not with previous QGIS versions. I work with LTR version for my work so upgrade to current version (2.18.3) is not possible for now.
Selecting features tools work perfectly with shapefiles but not with tables loaded from a PostGIS database via DB Manager. Clicking on a feature (or englobing it with a selection rectangle) does not select it either visually or within attributes table. The only workaround I found is to select a feature from the attributes table (select a row) and therefore, the feature is coloring on the map canvas...
I checked this post which is not exactly the same issue (shapefiles) but the solutions didn't help me.
Eventually, I can't see any errors when I load the table or within DB Manager UI: