From the text:
Men were screaming. Jon reached for Longclaw, but his fingers had grown stiff and clumsy. Somehow he could not seem to get the sword free of its scabbard.
-A Song of Ice and Fire: A Dance With Dragons, Chapter Sixty-One (Jon).
This is unfortunately all we have, since the explanation is not out yet.
Stiff and Clumsy
It is brought up many times in the books that Jon has to "flex his sword hand" to keep it active, and as brought up by Memnoch, his fingers must have gone stiff.
And then GRRM throws in a little bit of a spanner in the works. The phrase 'somehow' alludes to something else holding Jon back.
- Melisandre: could have cursed his hand, but it would defeat the purpose of her warning him of "daggers in the dark"
- Warging: into Ghost is a possibility; his mind was already half gone, so it couldn't control his body very well
- Poisoning / Intoxication: could have stiffened his reflexes, although Jon did not have anything to drink that night
- Someone: holds the sword in the scabbard. Remember, there were multiple attackers
- Not just a scratch: Jon is injured, severely so. Most people believe that the first couple of blows were really light/weak because Jon puts up a strong fight#; but he could easily have been more injured than we think
We will not know the true answer until Winds of Winter or beyond.
# I for one subscribe to the "Jon the Berserker" theory, which outlines that for some reason (Wolf Blood or
he has moments of near super-human strength.