I have never before had trouble obtaining this item. I always unequipped everything and then avoided fights on the way back to Bastok.
This time, while following the same method, the Tear slipped through my fingers about 10 times in a row (it spawns in about 5 different locations along the stream).
One thing that's absolutely necessary...!
Make sure your inventory isn't full :( I removed the "Vial of Fiend Blood" from my inventory and my next attempt was a success.