I already answered to that, Anwar. Of course the torpedo explodes when it hits the hull. What else would you expect it to do? Just because it explodes doesn't mean it did any damage. They exploded in the second half of "Q Who" too, yet Worf specifically announces immediately after that they had no effect.
Now consider this.
Based on special effects alone, we know that when Starfleet weapons are at their full effectiveness (as in, not adapted to), they can leave gaping holes in a cube:
But in the scene you mention, despite your claim that they lost their adaptive ability, the only leftover evidence that they were hit at all was this pathetic display of green sparks (which dissipated soon after):
So Anwar, if the Borg lost their ability to adapt to torpedoes in "First Contact", why don't I see any gaping holes in this shot? Or any discernible structural damage at all, for that matter? If they were constantly under attack for hours and unable to adapt, shouldn't they be completely destroyed by this point?
Now consider this.
Based on special effects alone, we know that when Starfleet weapons are at their full effectiveness (as in, not adapted to), they can leave gaping holes in a cube:

But in the scene you mention, despite your claim that they lost their adaptive ability, the only leftover evidence that they were hit at all was this pathetic display of green sparks (which dissipated soon after):

So Anwar, if the Borg lost their ability to adapt to torpedoes in "First Contact", why don't I see any gaping holes in this shot? Or any discernible structural damage at all, for that matter? If they were constantly under attack for hours and unable to adapt, shouldn't they be completely destroyed by this point?