It sounds like it was a failure to stay in place rather than failure to inflate.

"Within a day of US Sailing releasing a report that concluded that four out of five Spinlock Deckests failed to work properly in a fatal sailing accident earlier this year, PS testers were in the water with a Spinlock Deckvest (5D, 170N, Pro-Sensor inflator), trying to figure out what might have gone wrong. The new Deckvest was among several racing PFDs we looked at in the July 2013 issue, and at that time, we found no significant problems with it. It and the Deckvest Lite earned Recommended ratings. The accident report re-emphasizes what we’ve said several times before—inflate and try your PFD in the water as soon as you buy it. Make sure it keeps your head afloat without riding up around your head and neck. Learn how to service it, and adjust it for ideal fit. If it doesn’t fit, send it back and try another model."

... http://www.practical-sailor.com/blog/Report-Cites-Problems-with-Spinlock-Deckvests-11226-1.html