As everyone anxiously awaits the arrival of their online purchases the postal carriers are very busy processing the packages for delivery. During the busy holiday season, packages shipped as First Class or Priority Mail can have extended delivery times. It is common for the online tracking to be frozen for a few days or even miss its estimated delivery date. Please don't worry though, your items will still arrive :)
If you absolutely NEED it to arrive in the next few days you should purchase Priority Express. Yes, it is more expensive but this is the only shipping method that guarantees the delivery date will be accurate.