Flushing has been shut off. The quenching connection is not operating. The cooling jacket is clogged or not functioning for some reason. The discharge or suction recirculation line is clogged. The barrier fluid has stopped circulating in a dual seal application or you are using oil as a barrier fluid.
Oil has a low specific heat and poor conductivity making it a poor choice as a heat transfer medium.
If you must use oil as the barrier fluid you may have to forsake convection and go to a forced circulation system or a pumping ring. Silicon carbide is better than 99.5 ceramic. A wrong installation measurement. The impeller was adjusted after the seal was installed. Any pump impeller that adjusts against the back plate has this problem.