Employees should always check their pay stubs to make sure they receive the correct rate of pay for the correct number of hours. If you find an error, take it to your employer immediately. Often, they will correct the issue. Your collective agreement may contain language that requires them to pay the difference or a portion of the difference within a certain time frame.
You should also inform your Union Steward of any pay issues so that they can track it. Other employees may experience the same issue and they'll need to know if there's a bigger problem.