This is particularly true of a fretsaw, where the tension is limited more by the blade attachment than by the frame, which brings us to...
Fretsaw blades weren't really designed to be used at high tensions. There's only so far you can go with a simple friction-based retention mechanism like that. If you did somehow manage to tighten the plungers down far enough then the next thing to fail would probably be the blade itself. With that said you should probably make sure there isn't any grease or oil on your blade or plunger ends that could be degrading the coefficient of friction.
If you need that sort of tension then a coping saw is arguably a better tool for the job.