RX570 Hashing low

Hey yall hoping for a little help, I have a rig with 4 rx580s and 4 rx570s all are bios modded. 7 get about 30mh but one is only at 24.5 any idea as to why it would be so much lower than the others? Any ways to fix it?

you said modded so im assuming memory timings. Check to make sure the timings are for the right memory type. If they are try different timings. Polaris Bios editor has 2 straps for most memory types. If you are undervolting it could be the card wont operate at the low voltage. It could also be the OC settings. Some cards just wont be able to handle settings another card can.