Why doesn't the ETF follow the real index?

I have bought an ETF tracking the **Nifty50**. The ETF is. Over more than a year, the Nifty50 went up by **10%**, my ETF went **DOWN 10%**, and it made the total gain of my portfolio close to 0 canceling out all other gains. Why did the ETF do disastrously while the index it follows went up?