You didn't mention the dose you were prescribed, or how many doses you have taken.
Darvocet contains two ingredients -
propoxyphene and
acetaminophen - the strongest being the propoxyphene which is a narcotic analgesic.
Propoxyphene takes a little while to work effectively (hours) and the effects last much longer than acetaminophen, so it will work better with time. Hang in there, and if things don't improve call your doctor. He may be able to adjust the dose, or prescribe something stronger.