While you wait for answers, you might be interested in some threads from the FAQ section on "The Decline of the Spanish Empire":

• What happened to Spain to cause its decline? - 2 comments, over 5 months old.
• A flaired commenter talks about the decline of Spanish power primarily from an economic perspective and discusses the effects of the influx of precious metals on the Spanish economy.
• What are the reasons for Spain's downfall as a world superpower after the 17th century? - 3 comments, over 1 year old.
• A commenter points to events and economic conditions dating before and up to the very start of the Spanish Empire which undermined it's later efforts.
• Why did Portugal and Spain fall so far behind the other major European powers in the Industrial Revolution? - 17 comments, over 9 months old.
• Most commenters in this thread make an economic argument for the decline of the Spanish Empire, though one makes a very pointed argument for war as the main reason for the decline.
• What were the causes and consequences of the decline of the Spanish Empire? - 6 comments, over 6 months old.
• A commenter here talks about economic problems and issues that went on up to the very end of the Early Modern period and the transition into the Modern with the disastrous conclusion of the Napoleonic Wars.
• How and why did the Spanish Empire collapse all of a sudden? - 13 comments, over 9 months old.
• This thread discusses the financial and military strain of maintaining the farflung holdings of the Spanish Empire which combines with the above answers to give a greater understanding of the whole apparatus.