iTunes on my Mac Pro won't let me drag a playlist to my iPhone 4s. Why not? I do it all the time from my MacBook Pro.
Is there enough capacity? On the summary page for your iPhone in iTunes is "Manually manage audio and video" (located in the options section at the bottom) checked or unchecked;
You do it all the time with what from your MBP?
You have manually manage music and video selected under the Summary tab for your iPhone sync preferences with iTunes?
Retrieving data ...