Investigative mind is fine,

Based on what you saw it seems some property of the wood is interfering with the spell a little. You can tell that the wood itself is faintly magical, so that might be it. You could adjust your spell to take that interference into account (would take about a day of adjusting and testing to get it right, no further rolls required as you've got a decent enough grasp on what's going on), or increase the power put in.