Problem with RAM Bay

I finally added more RAM into my power hungry mac. Originally it came with 2x1GB Dimms, the first installed in the top bay and the second in the bottom as it should be. The new RAM I got was 4GB, 2x2GB blocks. I inserted the first in the second available bay and the other in the bottom second available bay. When I booted up the mac it didn't recognise the new RAM. I thought perhaps I made a mistake bought the incorrect RAM. I was pretty sure I didn't though. I double checked and yes it was the right RAM. I took out the original RAM (the 2x1GB) which was in the first slot and put the new RAM in. When I booted up again it recognised the new RAM. For some reason it doesn't want to work in the 2nd Bay. So put back the original RAM in the first slot and the new RAM in the 3rd Bay where it all works and have 6GB. I am concerned why the 2nd slot isn't recognising the RAM. I have done hardware tests and nothing is coming up as not working.
Can anyone advise ? The mac is still under warranty.

Warm festive regards
Sophia