This is kinda odd to answer my own question, but I want to state my findings here so that it might be helpful to others!
What I've figured out is, footer
was missing in my page-templates
! As most of the wordpress plugin's javascript
files are being called through footer
so if you miss <?php get_footer(); ?>
in your page-templates
then you may get such error like TypeError: jQuery(...).nggShowSlideshow is not a function
! I've missed footer while working with custom page-templates
.