Chipotle Mexican Grill (CMG -2.80%) stock hasn't been a hot buy with investors for some time. In the past year, its shares are up just a modest 10%. That's well below even the S&P 500's gains of more than 20% over that same stretch. And while the stock's valuation isn't cheap, in the past, investors were willing to pay a premium for it simply because it was such a great growth business to invest in. Its price soared so much that the company did a massive 50-for-1 stock split last year.

But the enthusiasm in the business looks to be waning. The stock isn't surging anymore, and it's now trading below where it was when it split its shares. What's wrong with Chipotle, and should investors be worried?

Same-store sales have fallen for a second straight quarter

Chipotle's impressive growth over the years has captivated investors and convinced them that it's a solid stock to own. But recently, cracks have been showing in the business as demand looks to be slowing down. Here's a look at the company's revenue growth and its comparable sales growth rate in each of the past four quarters.

Data source: Company filings.

For a second consecutive quarter, Chipotle's comparable sales growth rate has declined, and for the third time in four quarters, it has been below 10%.

Comparable sales growth is an important metric for investors because it shows organic growth, and how much revenue is rising at restaurants that were already opened in the prior-year period. Opening new restaurants can boost revenue, but that's as a result of entering new locations and markets. It's not as useful in gauging whether demand is truly increasing or not.

The slowing comparable growth numbers are a concern because that could make it difficult to justify the restaurant stock's high premium.

Chipotle isn't a cheap stock

Shares of Chipotle trade at more than 50 times the company's trailing earnings. This can be justifiable if a business is growing at a high rate. But if things are slowing down, that can lead to investors pursuing other growth stocks instead, which may be growing at faster rates.

CMG PE Ratio data by YCharts.

Chipotle's stock is trading in line with its five-year average, but investors do appear to be thinking twice about it right now.

What adds to the risk is its exposure to tariffs. President Donald Trump has threatened to impose 25% tariffs on Mexico (currently on a 30-day hold), which could affect Chipotle's earnings, as the company imports half of its avocados from there. Management estimates that the tariff could increase its cost of sales by 60 basis points.

Can Chipotle's stock bounce back?

Chipotle's business is slowing down, but economic conditions also aren't great, so a slowdown shouldn't come as a huge surprise. The company is still a great option for growth investors, as it has a long-term plan to operate 7,000 restaurants in North America -- that's around twice the number it has open today.

The short-term headwinds facing the company aren't reasons to turn bearish on the stock, especially if you're planning to hold on to it for years. If the stock falls lower in the weeks and months ahead, which may happen, that'll only make it an even better buy. There may be bumps ahead for Chipotle, but the business isn't in trouble, and buying and simply hanging on to the stock can be a great move for the long haul.