check the top left corner of the screen, you should see two numbers, like 8/10 or something. the first number is how many goblins you have, the second is how many you can support. if both of those are the same, or very close together, you might not have enough room for new goblins, try building more warrens.
if that's not the case, then i don't know. found i was only getting five to two new goblins every year after the first.