Some servers restrict iron drops from iron golems to prevent the building of giant automated farms, or restrict any drops from environmentally-killed monsters, again, to prevent the creation of giant automated farms.
This might be done for gameplay reasons (no supplies of infinite free stuff) or for server lag reasons (no concentrations of hundreds or thousands of monsters collected in a tiny spot).
Docm's advice is about the distance between two villages.
Your target is to build one village and another one far enough from the first to be counted as another and not the same village.
As the distance of your villages is 75 and the minimal distance between villages needs to be at least radius+32 Blocks, while radius describes the distance between the first village's center and the furthest door of it, your villages shoul be far enough from another.
There are now two error sources:
Either you have got any door between these two "villages" which connects them to one large, or you are too standing too far away from the not-working platform.
In the first case, destroy the doors in the middle.
If you haven't got any doors in between these villages, just try to play/stand/run whatever between them.
Depending on the servers configuration there is a limit of blocks that may be between you and the mobs (usually about 60-80 blocks). If you are too far away the mobs chunk stops working. This means, the mobs are not running around, not spawning anything and for sure not breeding anymore.
So, to get your farm working properly, make sure that all chunks of the farm are loaded.
Which you may either achieve by being between them or putting one of your friends at he bottom spawn layer and yourself at the upper.
Depending on which plugins are installed on the Server there may be even a block or something called "chunk loader" but one of them at the bottom-spawn-area and your problem is solved.
Best Answer
You don't need to constantly produce villagers in order to have an effective iron golem farm. Two basic conditions need to be in place for an iron golem to spawn:
Villagers: Iron golems spawn if there are at least 16 villagers nearby. For every 15 total villagers, one iron golem may spawn (16 = 1 golem, 32 = 2 golems, 48 = 3 golems, etc.)
Houses: It takes at least 21 houses to spawn an iron golem. A "house" in Minecraft has a complicated definition; here's the simplified version on the Minecraft Forum:
That description is slightly misleading as the algorithm is more complicated. For more information, I recommend reading Marfagames' thread on the Minecraft Forum.
Generally speaking, with both those conditions satisfied, iron golems will spawn. I encourage you to read up on the specifics if you wish to design your own farm. Lower down I also recommend a specific design.
Farming Tips
Having more than a greater number of villagers will not affect the rate at which iron golems spawn. The number of villagers only determines the iron golem cap.
If you drop the iron golems at least 5 blocks down from your farm, they will no longer contribute toward the iron golem cap, and more will continue to spawn.
A house only becomes part of a village if there is a villager within 16 blocks horizontally and 4 blocks vertically.
More Resources
JL2579 over on YouTube has made several videos on iron golem farming, and has designed the most effective golem farm I've ever encountered.
Micromancero came up with a farm design that inspirqed JL2579 (on YouTube).
Docm77 has done a lot of work with JL2579's farm on YouTube.
Note: I would love to post more links, but I don't have enough reputation. JL2579, Micromancero and Docm77's videos can be found easily via YouTube search.