‘If you talk to a man in a language he understands, that goes to his head. If you talk to him in his own language, that goes to his heart.’ ~Nelson Mandela
I have found that 98% of the people I meet all say the same thing, "I read and/or write Spanish well, but I can't speak a word." Thanks to many Spanish programs in schools across the US, learners are experts at worksheets, workbooks, and bookwork, yet they get very little opportunity to speak. On the other end of the spectrum, if you were lucky enough to go to a school that had a well rounded language approach, you may have spoken often in your class, but still feel to intimidated to speak.
In either scenario, not speaking Spanish, with some prior knowledge of the language, is a pure mental block. As with anything when you relax and find your motivation to speak- you will.
Here's a great article on speaking a language, with relaxation, focus and immersion, from the great people over at ZenHabits.com: http://zenhabits.net/how-to-learn-a-language-in-90-days/.