Memory help

I just bought new memory for the ole computer a HP Pavillion a220n. I made sure i got the right kind on the hp web page. the problem is after installing it the computer will lock up now. Then when i shut down it wont boot up. I assumed I had a bad chip so I switched them out putting the old ones back I checked the memory and both were working fine. I found out that one of the memory slots will not read the new chips but the other one will ? ? ? Have no clue what to do. I updated the Bios to see if that would help and it didn't.