UPS has dropped the ball for me more times than I can count, if you have the tracking number, look up a phone number for your local UPS depot, they may be able to clerify their issue.
Once I had some parts ordered, and the UPS site said they were undeliverable... someone was home all week, the lazy bastard driver just couldn't seem to realize that our house was on a corner lot, and we were regularly using the side door. The USPS carriers all understood, but this one UPS driver was just in too big of a hurry to deal with it.
After a few calls, the driver came out with the package on his own time and offered a personal apology. Nice.