1. Why does the valve stick? To be honest nobody knows why, it just happens.  Could be foreign material, moisture, manufacturing flaw, sign of the moon, well maybe not but then again ...

2. Why can't anyone work on the valve? The normal answer is parts are not available.  The true answer is lawyers and insurance.  If I work on a faulty valve I am responsible if it fails again.  Lawyers will pursue it and insurance companies have weighed the cost versus risk and said not to do it.