not getting full memory upgrade in precision

Hi,
In our lab, we have a Dell Precision 690 Workstation. We currently have 10gb of memory on the 6 out of 8 dimms. We recently bought a couple of 4gb units; however, the bios as well as os reads the total memory to be 10gb. In addition, the last two dimms where we installed the new 8gb shows 2gb each and they are not included in the total.
When I tried to take out 2 1gb units and replace them with the new 4gb units, it reads 8gb as the total memory (the new ones do not get included), and the dimm list shows those dimms to be filled with 2gb units.
In summary:
Initial Memory: 10gb Dimm map: 1gb/1gb/2gb/2gb/2gb/2gb/empty/empty
After installation, bios shows the total to be: 10gb BIOS dimm map: 1gb/1gb/2gb/2gb/2gb/2gb/2gb/2gb (right, it adds up to 14) Actual dimm map: 1gb/1gb/2gb/2gb/2gb/2gb/4gb/4gb.. Actual Memory: 18gb
Second trial.. BIOS shows the total to be: 8gb BIOS dimm map: 2gb/2gb/2gb/2gb/2gb/2gb/empty/empty (adds up to 12) Actual dimm map: 4gb/4gb/2gb/2gb/2gb/2gb/empty/empty.. Actual Memory: 16gb
The new units are the 4gb units (x2) and we've paid full attention to buy compatible units.
I would be very if you can lead us to a solution
Thanks in advance,