Steel importers are winning most of their requests for tariff exclusions for products they say they can’t find in the U.S., but the process is riddled with inconsistencies and frequent procedural changes, according to manufacturers and importers.

The Commerce Department as of Dec. 17 granted about 75% of the 19,000 requests it processed to exclude products from tariffs on foreign steel that took effect in March, according to a Wall Street Journal analysis of those applications. Exclusions issued so far cover 3.8 million tons...