It may not have been seated properly. Ram sticks are keyed and they only fit in the sockets one way. When properly inserted the fold down tabs at both ends of the sockets should snap upright.It's funny that the new one we added was ddr3 1600 but the machine did not recognise it.
There's also the possibility that you purchased dual channel ram when the existing stick is single channel (or vice versa). If the existing stick doesn't specify which type it is, my suggestion is to purchase a matched pair like this:
https://www.canadacomputers.com/product ... _id=060911
and then discard the existing stick after the new ram has been installed and is working properly.
Also, I think that in this instance, I think that it would be a good idea to go to an actual computer parts store and asking for their help rather than blindly ordering on Amazon.