U.S. companies are putting savings from the corporate tax cut to use, but only a fraction of it is flowing to employees’ wallets, new data show.

In the months after the December tax-code overhaul that lowered the corporate rate to 21% from 35%, dozens of companies such as Walmart Inc. and FedEx Corp. announced one-time bonuses and wage increases for hourly workers. Those moves earned praise from the Trump administration as evidence the cuts were quickly reaching many Americans.

Now,...