I don't know of any single-bolt post that tightens the ability to to rock before tightening the rails enough that sliding would be impossible, even with greased rails. I suspect that the top plate is bottoming out on something (a misplaced washer) and preventing effective clamping on the rails. I'd take the post apart and make a rail sandwich with the clamping plated (no post or bolt) and look around for any parts making contact, or near contact. Likewise, I'd check that the bolt's thread is OK and it'll thread in well beyond the point it needs to with the clamp attached.

If I didn't spot an issue, and still had problems, Id find something about 1/6" or so thinner than the rails, maybe a piece of wooden dowel and see if the post can clamp that effectively on two sides. By now you should have spotted the issue, and made an adjustment, either shim the saddle rails, or used a traction compound to improve the grip.