Luhn Algorithm

Hello, I'm having trouble with the Luhn Algorithm problem. For some reason my algorithm works all for two test cases. In fact in one such case my code even adds an extra number. Can someone anaylyis my code and lead me in the right direction

def luhn(digits):
    tot = 0
    for i in range(len(digits)):
        if (i+1) % 2 == 0:
            curr = (int(digits[i]) * 2)
            if curr >= 10:
                curr -= 9
            tot += curr
        else:
            tot += int(digits[i])
    return tot


for _ in range(N):

    digits = list(reversed(list(input())))
    try:
        idx = digits.index('?')
        digits[idx] = 1
        result = luhn(digits)
        for i in range(2, 11):
            if result % 10 == 0:
                break
            digits[idx] = i
            result = luhn(digits)
    except ValueError:
        result = luhn(digits)
        i = len(digits)-2
        while i >= 1:
            if result % 10 == 0:
                break
            digits[i-1], digits[i] = digits[i], digits[i-1]
            result = luhn(digits)
            if result % 10 == 0:
                break
            else:
                digits[i-1], digits[i] = digits[i], digits[i-1]
            i -= 1