It is not a problem in the installation of QGIS 2.18.11 and version 2.3.2 on your machine, but actually not all SAGA tools existing in SAGA standalone program are integrated in QGIS SAGA processing tools. There are many tools in SAGA standalone program such as slope stability (completely not included), Lighting (3 tools in QGIS, but 7 tools in SAGA software), and many others which are very useful and not integrated in QGIS processing tools.
Maybe it is not a convenient way, but the easiest route you can follow is to work with both software to get the results you want. This is the way I do if I need to work with a SAGA tool that does not exist in QGIS processing toolbox.
Integrating all SAGA tools in QGIS may require to do some python programming as there is a python wrapper located in
C:\Program Files\QGIS2.18\apps\qgis\python\plugins\processing\algs\saga if you download the QGIS installer (thanks to AndreJ for pointing that), or in
C:\OSGeo4W64\apps\qgis\python\plugins\processing\algs\saga if you download QGIS from OSGeo4W. Therefore, it will be a lengthy process if you want to do it by yourself.