Are you undervolting?
Also check your intensities on the miner software. Miner software can affect how far you can clock
Unfortunately all vegas are different. Every single one of mine has a different max clock on memory.
I dont think it have anything with undervolting or intensities, because if i am able to use Mem clock up to 950 if its only 1 vega per rig... But if i add another vega, i need to lower both Mem clock to 935... and if i add another vega... i need to lower all vegas again to 920, and if add another vega.. i need to lower all the vegas to 910.. thats way its odd....
and it happen to both rigs that i have...