I worked out what the problem was. The target entity was closer to the source entity than the entity I was trying to pick. As a result LinePick couldn't find any entities between the source and target entities and thus the Picked values remained the same as they were from the last successful LinePick (ie. the one with a different target entity).
Doh! - *slaps self on forehead*
Doh! - *slaps self on forehead*