My guess is that the first time you attached the foot straps, you didn't come back and re-tighten them after a few sessions. It takes some time for the material to compress and after a few sessions, the straps become a bit lose. They usually twist some, which is the warning that they the screws are not tight. If this is what happened to you, then they may have bent as a result.
Next time, re-tighten the screws after every couple of sessions, until they stay tight.
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