less doesn't work

Recently I've noticed that less command doesn't work.

$ ls | less                                                        
CertificateEntreprenuership101.pdf
Engineering Software as a Service_ An Agile Approach Using Cloud Computing_B00CCEHNUM copy.azw3
Photos.zip
Raspberry_Pi_Education_Manual.pdf
chef-v100
chef-v100.zip
node-webkit
postgresql-v400 2
postgresql-v400.zip
rd1_ua
rd1_ua.zip

As you can see it didn't open less as viewer. It might be some global setting but I have no idea what might it cause.

Update.

$ which less
/usr/bin/less
$ echo $LESS                                                           
-F -g -i -M -R -S -w -X -z-4
less <somefile> - works as expected

From man less:

   -F or --quit-if-one-screen
          Causes  less  to  automatically  exit  if  the entire file can be displayed on the first
          screen.

Remove the option from LESS to avoid this.