Hundreds of U.S. stocks have doubled in 2020, and recently, I pointed out three stocks that I think could double again next year. But naturally, plenty of this year's big gainers won't be running more victory laps in 2021. 

Among the stocks I don't see doubling again next year are Jumia (NYSE:JMIA), GameStop (NYSE:GME), and NIO Limited (NYSE:NIO). Here's why. 

A person jumping from the 2020 cliff to the 2021 cliff with the sun setting in the background.

Image source: Getty Images.

Jumia

Jumia has established itself as an early leader in Africa for e-commerce and payments processing, and some investors see it as a potential breakout candidate. The problem with ground-floor opportunities, though, is that sometimes the floor collapses and you find yourself in the basement. And then the sub-basement below the basement. 

Jumia stock has been nearly a five-bagger in 2020, but it hasn't earned its gains. Most successful e-commerce companies have thrived in the new normal, but Jumia's revenue actually declined in each of this year's first three quarters. Its trailing loss is larger than its revenue. 

Don't sleep on the potential for long-term e-commerce growth in Africa. The investing opportunities will come, but just not anytime soon. Jumia is struggling as it attempts to tackle the infrastructure, shopper reluctance, and fulfillment issues getting in the way of the eventual digital revolution in its core markets. 

GameStop

If you scanned through the list of the more than 200 stocks with market caps greater than $1 billion that have doubled in 2020, it's likely that the name GameStop would jump out at you. Wasn't the video game retailer left for dead at the strip mall? Wasn't this supposed to be the next small-box techie haven to follow RadioShack into Obit City?

Instead, its share price has somehow risen 169% year to date, despite a business model that's only becoming more dated with every passing Madden release. Net sales are declining for the third fiscal year in a row. It's been three years since GameStop delivered an annual profit. It's also been nearly two years since it last distributed a quarterly dividend, and those payouts are not coming back anytime soon.

The video game industry is in an upgrade cycle now with new Xbox and PlayStation consoles enticing customers to open their wallets this holiday season, but that won't keep them coming back to GameStop. Game distribution has largely gone digital, and that cuts physical retailers out of the food chain. A recent deal that will let GameStop capture some downstream revenue from the Xbox ecosystem will prove minimal and ultimately transitory. A lot of people have been betting against GameStop, and that helped fuel a short squeeze that sent the shares higher in 2020. Gravity should prevail again in 2021. Indeed, GameStop reports its latest quarterly results after Tuesday's market close, so the stock might not wait till next year to start accelerating back toward the ground.

NIO Limited

This will probably be the most controversial of my picks, largely because NIO Limited is the only one of the three that's actually growing right now. The Chinese maker of electric cars more than doubled its revenue in its most recently reported quarter. Unlike some electric vehicle companies that are still dreaming of selling cars or charging them up, NIO is doing it.

NIO cleared 12,206 vehicles last quarter, and it expects to sell as many as 17,000 in the current one. The problem here is one of valuation. NIO stock has been an 11-bagger thus far in 2020, but it's certainly not growing that fast. It's now trading for more than 30 times trailing revenue, an unsustainable multiple in the low-margin automotive world. NIO as a company will continue its growth in 2021, but the stock could -- and should -- take a breather.