The first question is when the leak starts. Is it present before the shaft turns, only while the pump runs, or after the pump gets hot? Those are different clues.
If it leaks while stationary, check where the liquid emerges. A leak at the gland, sleeve or a secondary seal may be an assembly, gasket or elastomer problem rather than a damaged face. Check the fitted part, its size and the installation record. Do not assume the new seal was defective.
If the pump holds while stationary but leaks when it turns, check the face contact pattern and the pump. Shaft movement, gland alignment, distorted mating faces and bearing condition can disturb contact. Confirm the seal setting and that any flush or barrier supply is available before running. If the faces were run without enough liquid, they may already have been damaged.
If leakage begins only as temperature rises, record the temperature at the seal, not just the process set point. Check cooling or flush flow, pressure and changes in the liquid near the faces. Heat marks are a clue, not a licence to change face materials without checking the duty.
Make the pump safe under your site procedure before opening it. Save the failed parts and photograph the seal as found. Tell us whether it leaked before rotation, during the first run or after warming up. That short timeline is often more useful than a general description of a bad seal.
Related: Seal Troubleshooting Guide and Why does the same mechanical seal keep failing on my pump?.