Thank you. What you will need to do is place the x8 dimms in the following slots - A1, A3, A5, and A7. as well as B1, B3, B5, and B7. Then place the x4 dimms in A2, A4, A6, A8 and B2, B4, B6, and B8.  Also, have you confirmed that you are not trying to mix Rdimms and Udimms? Let me know how it goes.
We have a PowerEdge T620 that came with 64GB ram, 8x 8GB 2Rx4 DIMMs We want to upgrade to 128GB and have ordered 8 Kingston KTD-PE313E/8G DIMMs  The Kingston RAM is 2Rx8. The user manual says you can mix x4 and x8 DIMMs but I have not been able to get it to work getting error PST0129 Memory is detected, but is not configurable. Check memory devices. The system works with either the new RAM or the old but not both I have enabled Advanced ECC and have tried various combinations including separating the types between processors but to no avail. Is it possible to mix 8 2Rx4 with 8 2Rx8 and if so which slots should I use?
T620 mixing 2Rx4 & 2Rx8 DIMMs