If I recall most common reason is that the post-crash fire burnt them up. There has been a bit of discussion going on in various places about where the ELT should be placed to limit the risk of this but most of the recommendations (in the tail seems to be the most common recommendation) only are suitable for Fixed wingers.
The other problem is if you go down in water - not just the ocean but also lakes, rivers etc. As soon as the aircraft sinks the ELT often loses its coverage.
I don't recall hearing of many other situations where it hasn't gone off..but there may have been.