EA Live Trading VS EA Strategy Tester


Ok, I created an expert advisor that uses a couple of indicators and its been working great but one day last week one indicator gave me negative signal when it should have given me a positive during live trading. I thought it was an issue in my code, I was upset but I knew I could fix it by back testing with the strategy tester. Now, when I ran the strategy tester with the same expert advisor, I did not get the same result. Could it be a bug in the indicator or are there differences between every tick based on real ticks? The EA worked successfully everyday for 6 weeks, I just don't know what could have happened. Thanks any help would be greatly appreciated.