Getting wrong output while using Monotonic stack to find previous Greater Index in array

Question

I am using Monotonic Stack for finding previous greater indexes for the elements in the list arr. I want to return a list with all the indexes that shows indexes of previous greater element. But for any input , I am getting output as arr = [-1, -1, -1, -1, -1, -1, -1, -1, -1, -1]. What is one thing that I am missing in the below code?

class Solution(object):

      def previousGreaterIndex(self,arr):

          stack = []

          previous_greater = [-1]*len(arr)  # create an aray to store elements

          for i in range(len(arr)):

              while stack and arr[stack[-1]] <= arr[i]:

                  stack_top = stack.pop()

                  if stack:
                      previous_greater[stack_top] = i

                  stack.append(i)
          return previous_greater

if __name__ == '__main__':
    arr = [13, 8, 1, 5, 2, 5, 9, 7, 6, 12]
    print(Solution().previousGreaterIndex(arr))

i even tried to tweak <= to < to change the monotonic stack to non-increasing but not getting the required result.