ram not used by system

[adamyoo]

i added 4 gigs of ram to my pc and everything went fine and the mobo and windows recognized it but windows says only 16 gigs usable(amount before upgrade)

 

[Tradesman1]

What mobo, CPU and model #s of the DRAM, mixing DRAM isn't a good idea, best to get full amount needed/wanted in a single package (then they are tested and better yet, guaranteed to play nice), mixing packages is a crapshoot, maybe they play, maybe they don't, even with sets of the same exact model