NEW YORK (TheStreet) -- Shares of Jack in the Box (JACK - Get Report) have slid 20% since the start of August. I think the stock will continue to go lower.

In the last year, Jack in the Box shares has been on a roller coaster. In the first quarter of fiscal 2015, its Qdoba restaurants reported same-store sales of 14%. As investors anticipated a blowout second quarter, they drove JACK to a 52-week high of $99. Then, when comps fell to 8.3% in the second quarter, the stock dipped. After the third quarter report showed Qdoba sales were up just 7.7% against last year's 7.5%, growth investors hit the exits. The stock fell into the high $70s. Jack in the Box closed Thursdy at $77.

While a same-store sales number of 7.5% for a quick service restaurant is very good, it's just not enough to sustain a high stock multiple on a company that has single-digit revenue growth.

In the third quarter, Jack in the Box grew revenue by just 3.2% to $395.5 million. Next quarter, investors are looking for 2% revenue growth. Add it all up and the full year looks to be up just 3.6%. Why pay 29 times forward earnings for that?

While revenue growth is stuck in the low single digits, earnings are up sharply because of an aggressive stock buyback program. Since 2009, the company has bought back $956 million worth of stock. Last year, Jack in the Box bought back a staggering $319 million worth of stock at an average purchase price of $56.63. While earnings "grew" 23% in fiscal 2014, revenue fell 0.4%.

The company likes to talk about its menu innovations and a new restaurant format but I think the stock will continue to go lower. The company hasn't convinced me the can actually grow its top line in any meaningful way. Rennovated resturants are years away from increasing revenue.

Revenue fell 0.4% in 2014, 24.9% in 2013 and fell another 10% in 2012. How can that be a considered a growth story? The Qdoba restaurants have been riding Chipolte's  (CMG - Get Report) coattails for the last two years. In 2013, Qdoba consistently had same-store sales between 1.1% and 2.3%. Qdoba changed its menu and started producing same-store sales between 7% and 7.7%. But now Qdoba's same-store sales can't escape gravity. They are coming back to earth.

I think its clear Jack in the Box' stock price is held hostage by Qdoba's same-store sales figures. If management can't reaccelerate Qdoba, I think momentum investors will continue to flee the shares. If I'm right, Jack will get stuffed back into the box.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.