Monday was a good day for the stock market, which continued to build positive momentum going into earnings season. The Dow finished up more than 200 points, and other major benchmarks posted similar gains on a percentage basis. Upbeat views about the positive impact of tax reform on corporate profits and a greater confidence in the U.S. being able to navigate geopolitical and global macroeconomic pressures buoyed investor sentiment. Yet poor news for some individual companies held back their shares. National Oilwell Varco (NOV 1.29%), Dean Foods (DF), and Acacia Communications (ACIA) were among the worst performers on the day. Here's why they did so poorly.

Sluggish conditions hit Varco

Shares of National Oilwell Varco dropped 7% after the oil-field services company reported that its first-quarter revenue would be less than it had initially expected. The company now believes its top line will come in at $1.8 billion, and Varco blamed reduced progress on new offshore rig construction and delays in deliveries of key equipment in pushing sales into future quarters. Moreover, it said that lower shipments of subsea production equipment also weighed on performance. CEO Clay Williams still thinks that industry fundamentals are improving, but Varco will have to prove to investors that the delays will create only timing issues rather than outright sales losses.

Complex network of pipelines converging at a central network node.

Image source: National Oilwell Varco.

Dean deals with a downgrade

Dean Foods stock lost 9% in the wake of getting a downgrade from a key Wall Street stock analysis company. Analysts at Goldman Sachs cut their rating on the dairy giant from neutral to sell and reduced their stock price target by 20% to $8 per share. Dean Foods has done a reasonably good job of cutting costs, but milk sales volumes have been weak, putting pressure on prices. The company also lost a key contract with a grocery store chain, and that could make it harder for it to hang onto its efficiency gains. With the milk producer having been under pressure for some time, today's news was just the latest hit that Dean Foods has had to take.

Acacia takes a trade hit

Finally, shares of Acacia Communications plummeted 36%. The company fell alongside most of its peers in the optical components industry in the wake of a decision from the U.S. Department of Commerce that could have a major impact on a key customer for the sector. Specifically, the regulatory agency said that it would block China's ZTE, a major telecommunications company, from importing components from U.S. companies for seven years. The news is bad for a variety of players in the space, but Acacia got fully 30% of its sales from ZTE, and so it took the biggest hit among its peers. Even if market conditions get better, it's tough to see how Acacia will be able to take full advantage going forward.