In addition to jammed canopies, I'd like to see other things preventing the pilot from successfully bailing, such as "trapped in cockpit by G-forces," "pilot wounded; unable to bail-out," and my personal favorite from Aces over the Pacific, "you've struck the tail as you bailed out...unconcious, you plumet to your death." That last one could be aircraft and attitude dependent.
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Rojo (S-2, The Buccaneers)