Iteration does not work for double characters?

Beginner here. I'm having problems with this task: accum("hello") should return "H-Ee-Lll-Llll-Ooooo". But what I get with my code is "H-Ee-Lll- Lll -Ooooo". It doesn't work for double characters. Is this because the iteration variable in "for i in s" "skips" over double "i's" or something? And do you have an idea how I can fix this? I'm not striving for elegant code or something, my goal atm is to try and make easily readable lines for myself
Thank you!
(Sorry if this is something basic, I didn't really know what to search for!)

def accum(s):
        s_list = []
        s = [ele for ele in s] 
        for i in s:
            sum_ind = ((s.index(i)) + 1) * i
            s_list.append(sum_ind)
        s_list = [e.capitalize() for e in s_list]
        s_list = '-'.join(s_list)
        return s_list

Here's a way to do:

def accum(stri):
    p = []
    for i, s in enumerate(stri, 1):
        p.append((s*i).capitalize())
    return '-'.join(p)

accum('hello')
'H-Ee-Lll-Llll-Ooooo'

Take a quick read about: enumerate

Here is one way:

def accum(s):
    return "-".join( (c*i).capitalize() for i,c in enumerate(s,1) )

yields:

'H-Ee-Lll-Llll-Ooooo'

I think you could solve this easily with enumerate:

def accum(s):
    r = []
    for i, letter in enumerate(s):
        r.append(letter.upper() + (letter*i).lower())
    return '-'.join(r)

As mentioned in many comments, we can give a short explanation of working of enumerate here.
As per your requirement, given a letter from a string, you find its index (position). Then you first make the caps of letter and glue it with index-times small letters.

So you need a counter which keeps track of the position of letter, so we can do something like this (A DUMMY, SLOW EXAMPLE):

def accum(word):
    ans = ""
    for index in range(len(word)):
        letter = word[index]
        ans += letter.upper() + index*letter + "-"
    else:
        word = word[:-1] #Remove trailing '-'
    return word

BUT THIS FUNCTION IS EXTREMELY SLOW. BECAUSE IT USES SIMPLE STRING ADDITION AND NOT OTHER PROPER METHOD.
That's why people ask you to use enumerate. In short it keeps track of your indices.

for index, name in enumerate(["John", "Lex", "Vui"]):
    print(name, "is found at", index)

That's it !
{Im not writing the answer you wanted, as almost everyone provided the best answer, they could, my aim was to explain you the use of enumerate and other slow methods for your problem}