I had difficulty, only one card is read, the computer says i slot available. Have fitted several times and exerted pressure to ensure contact but still only one card read. Any ideas please?

I have just fitted the two 8 GB rams but despite refitting several times my computer is only reading one card. It simply says 8 GB memory and one space available. The cards slotted tightly and clicked in position. I have tried exerting a little pressure on the cards to see if there was not good contact but they appear connected. Any ideas please?