Hi Anne,
It's really hard to break a pinned blade, they typically are a size 7 or larger when compared to a flat blade, it sounds to me like you are simply pushing too hard and too fast. If the blade is bending to the back when you are cutting, slow down. If your arms, get tired or sore, your'e pushing to hard.
Scroll saws are about precision, not speed. Since your blades are breaking in the middle, you are putting to much stress on it as you feed it through, slow down and let the blade do the work, don't try to force it. As long as the blade goes up and down, it's not a saw issue, it's a user issue. Yes flat blades will work better for you for intricate work, but for cutting straight lines on a poplar leg, the blades you are using should perform fine.
Remember even pinned blades are small, and cannot be treated like a band saw or a table saw, you will get the hang of it with practice and you will begin to feel the amount of force needed to make your cuts.
Oh....and loose that hold down bracket, they are useless.