Worshipping and Doubting

In Matthew 28:16-20, we usually miss verse 17: "They worshipped him, but some doubted." What did the 11 disciples doubt during this post-resurrection appearance of Jesus? Did they doubt that he had been resurrected? Or that he was the Messiah, the Son of God? Or did they doubt themselves and their ability to carry on after Jesus left them? The interesting point in this is that some of the same disciples who worshipped him, also doubted. What can we learn from the disciples' struggle in the aftermath of the resurrection?