Whys isn't my music from my Macbook synching to my iPhone 4S?

For the past few days, anytime when I plug my USB from my iPhone 4S into my Macbook pro, the downloaded music on my computer does not synchronize to my phone. It stops at step 5 of 5 and never loads anything. Also today I also noticed on my phone that all the music that I downloaded from the internet is not playing on the phone and automatically skips over until to comes to a purchased song from iTunes. Some of the songs have a red round stop button next to it. I have never had this problem, can somebody Please help me out on this?

It can be due to few differnet problems
First check on you macbook in itunes whether all songs are there in itunes or not
Of it is there and see whether sync all music option is checked
And also if there's storage available on your iphone
The red stop button maybe that sync or downloading of the song was not completed
Try this few steps

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