A report in the Washington Post, citing congressional aides, says as of Tuesday, IRS officials had been unable to reach 9,000 workers and another 5,000 claimed hardship exemptions. The recalled workers, many of whom work in the tax processing and call centers, were being brought in—without pay— as the IRS prepares for tax-filing season, which begins next week.

The 2019 tax season was already going to be a complicated one as people adjust to the new tax laws. Only about 35% of calls to the IRS call center are currently being answered, with the average time jumping to 25-40 this year from 7-10 minutes last year .

Taxes are still due in a government shutdown, but refund checks will either be delayed or won’t go out until the government reopens. That’s likely to increase pressure from taxpayers on the Trump administration and congressional leaders to settle their differences, especially since the average refund was predicted to be much higher this year.

The number of employees who have not returned to work could have shifted since Tuesday, The Post notes.

Staffing issues are becoming more prominent in several government branches as the shutdown drags on. On Friday, the Federal Aviation Administration was forced to temporarily close LaGuardia airport, one of the nation’s busiest hubs, because of a lack of air traffic controllers.