Reply To: why i cant plane this piece of oak ?

I believe the most likely issues are that either the board is convex along its length (in which case the plane will only bite at the ends) or the plane iron has a bevel that is too steep. If you sharpen by hand, or even if you are not careful when using a strop, it is possible to change the angle right at the tip of the blade to beyond 45 degrees, after which the tip will no longer meet the flat surface of the wood. The plane will still work in pine as the blade can bite into the soft surface by a fraction of a mm, but it will fail on harder woods. If this is the case you can restore the bevel to 25 degrees using a honing guide or similar, and then evaluate your sharpening technique to figure out what is causing the angle to steepen.

Hope you manage to resolve it one way or the other.