Why all of the items are not appending to list b after adding pop method

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Python

Python3.6

a = [11, 22, 33, "aa", "bbb", 9, "cccc"]
b = []
x = 0
for i in a:
if isinstance(i,int):
b.append(i)
a.pop(x)
x = x + 1
print(a)
print(b)

What I have tried:

before adding pop method all of the items are appending but after pop they are not