ITunes won't download all the songs to my iphone 6s after transffering my list of songs from my old macbook to my PC

I transferred my list of songs from my old Macbook to my PC with iTunes. For some reason, iTunes won't download ALL the songs to my iPhone 6s from my PC. For example if I want to download a whole album it will only download few songs from that album. Anyone know how to fix this problem? I also noticed how long it takes to download just few songs from my PC to my iphone 6s which is really frustrating.

Double check if the songs on your iTunes library have same folder location as before. Sometimes iTunes does not indicate which songs it can't find until you select them. Songs in your library that iTunes can't locate, won't be synced to your iPhone, iPad or iPod.
When syncing music to iPhone/iPad/iPod with iTunes, go to the Music tab - is "Entire music library" selected? If not, go to Music (in your iTunes library), click on the check mark next to the "Name" column header (to sort by checked music) and scroll to the bottom, or any unchecked music won't sync.

"ITunes won't download all the songs to my iphone 6s after transffering my list of songs from my old macbook to my PC."

When you say "download", do you mean "Sync"? When you say "list of songs", is that a Playlist in the left frame of the iTunes window? My answer assumes "Yes" for both of those questions.

Maybe some of the songs are missing. Double-click each songs in the iTunes Playlist one by one to see if they all play. If you know for certain exactly which songs are missing from the phone, be sure those missing songs are listed in that one special Playlist (I named my special playlist "Phone songs").