Maybe the thorn is in a seam, thus not that easy to feel/spot ?
I have seen one guy getting his kite in a thorn bush.
The bladder could be repaired easily, although A LOT of tape on the numerous holes.
But even after having looked after thorns in the leading edge and felt with hands inside, and washing with a hose - the bladder still got punctures after reinsertion, so he dumped the kite and bought a new one (it was not a new kite...)
Can be tricky apparently...
It will not help you, I know, but would just share it CAN be a problem, and not only you who has experienced this