Can't transfer all music to iPhone 4 running on iOS 8 latest

Question

In the process of changing music on my iPhone 4G, I noticed that not all music I had intended to transfer made it. On the iPhone the music does not show up at all but on the directory on iTunes on my computer, the songs are listed, but in grey and preceded by a dotted circle, i.e., they have not been downloaded to the iPhone. At first I thought it may have to do do with me recently having changed my e-mail address and iTunes ID, but there is no rhyme or reason to why some songs are not being transferred. Having said that, it does appear that if one song in an album does not transfer, the entire album doesn't transfer no matter whether the music was purchased prior or after the ID-change. Only about 10% of the files I moved are affected.
I erased all the music on the iPhone and started from scratch again and I reorganized the iTunes library to no avail.
Can anybody enlighten me as to what is going on here and how to work around it?
Thank you!
Tom
Tech details:      iPhone           4S; iOS 8.1.3.; 5.3 GB free space
                         computer        Mac Pro 2 x 2.4 GHz Quad-Core Intel Xeon, 16 GB memory, OS 10.10.2 

Answer 1

Thanks. No, I'm not sure. I remember that I may have physically connected my IPhone at some point to another authorized computer that had songs from other sources and songs originally downloaded from another old Itunes account that I no longer use. I think those songs did sync from the other computer to the Iphone. Is that possible? Is there any way to get those songs from my Iphone to my computer?
Thanks.