Can sperm ever disappear after an initially successful reversal?

Yes.  It is estimated that sperm may disappear in 10-25% of cases after an initially ‘successful’ reversal.  This is usually due to obstruction or scarring at the reversal site.  Sometimes it can occur because sperm have leaked out of the join (the anastomosis site) and cause a reaction known as a sperm granuloma that leads to scarring of the tube.