Since Thranduil is so vigilant with keeping foul things from his borders wouldn't it be hard for Sauron to get any intel from his spies about what the elves know about Gollum and his connection to the ring? Maybe his main motivation to spring Gollum would be to on one hand find out what the enemy gleaned from him, and on the other hand prevent the elves from getting any further knowledge through interrogations. Now it is known from the hobbit that thrushes have been known to speak so it wouldnt be outside the realm of possibility for an evil thrush to be used to inform Gollum to refuse to come down from the tree on the planned day. Or maybe Sauron could have had a few spiders gossip about the coming attack near Gollum's tree the day before only to give Gollum the impression he is eavesdropping on secret knowledge? This would dispel any suspicion he has of being captured by the orcs himself. Then perhaps later after being captured himself he finds an opportunity to break from the orcs in order to make for Moria since that place is closer to Dol Goldur than other ways over the misty mountains.