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I had a similar flakey problem and ended up communicating with Mathematica Support who suggested the following: 1. With Mathematica open, select 'Preferences...' from the 'Edit' menu. Then select the 'Evaluation' tab in the window that pops up....
Uh-Huh.  How to catch a Newbie. I guess I shouldn't have posted before actually examining the problem. I assume you've entered the derivative function incorrectly. In case you did enter your question incorrectly and are getting odd unexpected...
PROBLEM SOLVED: From communicating with Wolfram Support I got the message below. The first suggestion didn’t work but the second one (instructions at URL) did. According to the URL, resetting Mathematica to its default settings fixes a number of ...