I vaguely recall situations where students type an answer that WeBWorK doesn't understand (e.g. using extended characters, or with a syntax error). In these cases I believe I've seen it mark the answer wrong, but have trouble writing the answer to the past answers table. I would think something like this would be more likely than a database problem which only intermittently affects one table.
Have you checked the apache error logs? If WeBWorK is having trouble writing to the database then I would expect there to be errors somewhere.
WeBWorK Main Forum
Not all past answers being recorded?
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