Syncing all music onto iphone?

I recently got an iPhone 5 and on my iTunes I have all my music, about 1000+ songs, I've synced the songs on my iPhone but it only puts 300, when I try to add individual albums it says there's no songs that exist. My friend would send me albums through drop-box so I'm thinking that's the problem, is there any way that I can get all of the songs on my iPhone or is it too late?

Added (1). I did PKAP said but it still didn't work.

Your problem may be that iTunes can't find the song so what you see is the title, artist, and album. But there's no song. Have iTunes find the songs or go to iTunes>file>organize library> consolidate.
Note: This will take every song file on your computer, duplicate it if necessary, and put it all into a neat folder under iTunes. It will also do this for apps, ringtones, books, and movies. Then, you can delete the duplicates that are outside the iTunes Media folder where everything was just stored. This may take about an hour.

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