If an employee fails a fit test another size or model of respirator can be tested until they find a respirator that fits properly.
If an employee cannot pass a fit test on any available respirators they cannot be approved to wear a respirator. That employee must refrain from doing any work that requires the use of a respirator.
As an alternative option they can be trained on the use of a Powered Air Purifying Respirator (PAPR) which does not require passing a fit test to wear as long as the PAPR is used with a loose-fitting hood or helmet.