So, In the midst of a letsplay using Plenty-o-Biomes, I found out that pine wood slabs, when reclaimed from a construction, are mutually exclusive with the pine slabs that have not yet been placed in the world. With luck its a simple thing...

Could not reproduce this one i'm afraid. If there's some specific circumstances under which this occurs feel free to let me know, and ensure your using the latest version for 1.6.2.

This was submitted well prior to that version, while soon thereafter, the changelog did mention:

• Fixed some bugs with pine tree growth requirements and slabs

Someone may have already had a bead on that particular problem and didn't know about this bug