Try to remove the clamps and see if you can get the bolt to thread in without a clamp in place. I would be greatly surprised if they had different threaded sizes, though I would not be surprised if there was swarf from the manufacturing process that made it difficult to thread a bolt in. I would make sure there isn't anything in the threads.
Honestly, it could be that it takes 25mm bolts, and if you start the bolt and tighten it a little too much, it will push the opposite side up, keeping you from starting the bolt. If I were you, I would want about 2-3 full turns of thread engagement before it starts to really tighten the clamp, that way it doesn't pull threads out of the hole when you try to tighten it down.
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