A high share rejection rate can be caused by a number of reasons.
1. Firmware Issues
If you installed custom firmware make sure to check on the reject rate, also some firmware may get corrupted or needs to be updated. If you are having trouble contact the firmware developer or the ASIC manufacturer.
2. Machine Quality
Some ASICs have higher rejects than others due to the nature of the build. Even within the same manufacturer and same model ASICs are slightly different. So there is no standard across the industry and some machines are known for being a bit faulty.
There is a fair amount of data transmission during the mining process so your connection to our mining pool server has an impact on the rejection rate.
Run a ping test to our server like stratum+tcp://btc.luxor.tech:700.
Ping times should be below 100ms.
4. ASIC Temperature
The higher the temperature above the specified level, the more likely of rejected shares. Make sure to try to operate your ASICs in the operating band provided by the manufacturers.