BIOS update. Your motherboard has a BIOS (Basic Input Output System) that you can view by pressing a key when your computer starts before Windows loads. The motherboard manufacturer releases updated versions of the BIOS for their motherboards, especially when newer CPUs are released after the motherboard is manufactured. Sometimes a newer CPU won't work with a motherboard that isn't updated to the latest BIOS version.
Asus says the Phenom II X4 955 is supported since BIOS 1203. https://www.asus.com/us/Motherboards/M5A78LMUSB3/HelpDesk_CPU/
I'm not sure why their other BIOS versions aren't in ascending numeric order, but you can download and install BIOS 1203 from that link above if you need to install it. Installing it is another process that should be taken with carefulness, if the process is interrupted the motherboard can become useless.
Perhaps you should find out which BIOS version your motherboard is currently using. You can do this by entering your BIOS when restarting your system. It should say somewhere in there.