You are accidently nesting here. you set the index correctly but remeber your range goes from index/3 to len(aa) and not from index. so you actually start reading early on. 

Test results: 

Input DNA: GATGTTTATGGGCTAGTAG
Expected Output: ['ATGTTTATGGGC']
Actual (your) Output: ['ATGTTTATGGGC', 'ATGGGC']
FAILED - find all ORFs all frames