Solved- append a new atom upon finding specific atom

I've recently started working on LISP as part of my course.
I am trying to add a new atom after every search atom in the list, my code is as below -

(defun appendConst (OLD NEW L)
   (cond
     ((null L) ())
     ((EQ (car L) OLD) (cons OLD (cons NEW (cdr L))))
     (T (cons (car L) (appendConst OLD NEW (cdr L))))
))

This is working fine, but only adding the required atom in first occurrence. Please check example below -
Input: (appendConst 'a 'd '(a c e a m k))
Ouput: (A D C E A M K)
Required Output: (A D C E A D M K)

I tried calling the function recursively, but I can see that it doesn't work, as the no. of arguments are not properly being passed.

(defun appendConst (OLD NEW L)
       (cond
         ((null L) ())
         ((EQ (car L) OLD) (appendConst (cons OLD (cons NEW (cdr L))))  )
         (T (cons (car L) (appendConst OLD NEW (cdr L))))
    ))

Thanks.

You're close—it just wants to be flipped inside out a little bit—the second clause should be like this:

((EQ (car L) OLD) (cons OLD (cons NEW (appendConst OLD NEW (cdr L)))))