What led to Spain's economic decline?

The decline of the Spanish empire was brought about by many factors. Money was tight for the Spanish during the 17th century, despite that galleons filled with gold were sent from the Americas (though many were raided by pirates or were wrecked in storms). But conquering and forming an empire is one thing; maintaining it quite another. Often the natives of a conquered land are not happy with the conditions they're subjected to by the conquerors, just as often those conditions are brutal and unfair. So they rise up in arms, and that means that empire has to spend a lot of money on armies and weapons to extinguish it.