With so many rigs, you really should have a total hashrate of at least 900. Each GPU series RX 580 must give not less than 30 mh/s. Perhaps you have a problem with the firmware of the BIOS of the GPU. In order to achieve this speed, I upgraded the BIOS of my GPU. It seems to me that the main reason for this.
My total hash rate is 960, however my average is lower between 800-860. I have upgraded my BIOS and there reported hash rate is quite good 31-32 mh/s per card. The per cent of drop shares is also in range in my opinion beteen 1-4 % . I seems i need to do some more tinkering with the cards in order to up the average hash rate.