In the last major patch the wood yield from harvesting trees with a pick was removed. Now it only yields bark & resin (as it probably should be).
However, since then, the overall amount of resource yield is LOWER. It seems like as if when wood should have dropped, instead nothing is dropped. This leads to sometimes a full tree dropping resources only on one or two hits, rest of the hits yield nothing, and thus you could get like only 3 bark from the whole tree (with a starmetal pick).