If you or the super-user don't have a password, xlock will not lock.

  Under IRIX 4.0.x, xlock is set up by default to act as if invoked
  with the "-nolock" option, which tells it not to lock the screen.  If
  you want force xlock to lock, do either of the following:

  a) Use the "+nolock" option when invoking xlock.  This means
     "not nolock" (the opposite of "-nolock").  For example:

     % xlock +nolock

  b) Add the following line to your ~/.Xdefaults file:

     XLock.nolock:    False

  In IRIX 5.x, the default behavior of xlock has been reversed, so
  xlock does indeed lock by default.  If you wish to keep the IRIX 4.x
  behavior, invoke xlock with the -nolock option.