General Motors' (NYSE:GM) plan to ramp up production of high-profit pickups has hit a snag: Short supplies of key parts from Mexico. 

GM, along with the other Detroit automakers, restarted some of its U.S. assembly plants on Monday. GM restarted its pickup factories in Fort Wayne, Indiana, and Flint, Michigan, with just one shift of workers at each plant as it worked through new safety procedures. 

GM had planned to add additional shifts next week, in an effort to get more of its highly profitable full-size pickups to dealers quickly. However, parts from some Mexican suppliers aren't yet flowing in sufficient quantities to support the added production, the Wall Street Journal reports. 

Pickup trucks on a factory assembly line.

GM's assembly plant in Flint, Michigan, restarted on Monday. Image source: General Motors.

The plan to add shifts has been delayed until June 1, says the Journal. 

While governments in the U.S. and Canada gave the Detroit automakers and suppliers approval to restart factories this week, Mexico's government has been more cautious. GM said yesterday that it had received approval to restart operations at two of its four manufacturing complexes in Mexico, and that it would do so as soon as possible.

But it cautioned that its local suppliers may not be ready to support vehicle production in Mexico immediately. 

Those factories, in Silao, Guanajuato, and Ramos Arizpe, Coahuila, build some versions of GM's Chevrolet Silverado pickup and several of its crossover SUVs. 

Two other GM manufacturing sites in Mexico, in Toluca, Mexico State, and San Luis Potosí City, don't yet have dates to reopen, GM said.