Okay, here's the story. My brother and I are playing SMP (Minecraft 1.0, craftbukkit 1.0.1R2) and are really short of wool (land is overrun with wolves) so we need more consistent access to spiders.

We haven't found any spider spawners, so we decided to build a grinder in the form of a large drop (with sudden stop at the end), the same style we've both successfully used in the singleplayer SkyBlock Survival challenge.

The chamber is 18x18x2 (avoiding endermen), with 12 water source blocks in various channels (3 blocks high at that point) to sweep the mobs precisely to the 2x2 hole in the middle. That hole is 40 blocks above a platform.

We have made sure that we're at least 30 blocks away from the chamber, but well within a few chunk radius. Apart from exactly 3 mobs when the the thing was first launched, nothing is spawning. We're mystified.

The server is on difficulty 2. We're ensuring our distance is appropriate. We have switched in and out of creative mode a couple of times each to more easily recheck our setup, but I can't think that that would make a difference to whether mobs would spawn when we're both back in survival.

Help?!