I don't know about anyone else, but I found that the only Fen Striders and Spore Bats that dropped these items were those directly west of Telredor. No matter where else I went, I got no drops, but as soon as I killed only those west of Telredor, I got the drops right away.

This has occurred while doing this quest on two different toons on two different accounts. Coincidence? I think not.