Employer portion of Social Security

I'd first check the paychecks to see if the tax is accumulating. You can see the employee paid tax amounts by reviewing the paycheck details. For SS and Medicare, the dollar amounts usually match the employee withholding amounts, or are within a penny or two.

If it's not, then there may be something wrong with the YTD tax before the current period. If QuickBooks thinks the history YTD has to much tax, then it will stop accumulating on new checks until the wages catch up to the YTD tax amount.

To check, use the Payroll Detail Review report, which shows per employee, per tax, and then per paycheck, each line item including the tax amount currently on the check, the amount QuickBooks thinks is right, the difference, a running total, and whether or not a user overrode the amount from the original calculated on each paycheck. It's pretty useful for hunting down the cause of apparent issues like this.