Dell 755 SFF PCi-e issues

Bought two identical Optiplex 755 for running Minecraft for grandchildren . Bought two ATI Radeon HD 2400 Pro cards to upgrade so that Minecraft would run. One machine accepted the card perfectly no problems but the other refuses to recognise the card. The working machine is Bios A21, the problem machine Bios A22. All other settings in Bios and drivers are identical and both video cards work in the good machine. Lost for ideas? Can anyone help.