Change of mode does not activate the associated hook

[Heime]

Have made a minor mode named "ark" that uses mode hooks.
For a buffer with f90-mode enabled, the functionality in
the file avus-f90.el would be activated. This works well
when I open an file named "transform.f90".

But if I am in a lisp-interaction buffer and change the
major mode to f90-mode followed by "M-x ark-minor-mode",
the associated mode hook for f90-mode is not activated.

What is going on ?

(defun ark-gali ()
  (interactive)
  (cond
    ((eq major-mode 'f90-mode)
          (require 'avus-f90))

(defun ark-mode-hooks ()
  (add-hook 'f90-mode-hook #'ark-gali))

(define-minor-mode ark-minor-mode
  (if ark-minor-mode
       (ark-mode-hooks)
    (message "%s" "TEST")))