Constant work. Such online communities have an exponential growth pattern. So you probably have a certain amount of time after which you double.
This is obviously very apparent when you have 10k subs, and on the next day it is 10,1k. But when you are at 10 subs it can take days for the next to arrive.
Just go on posting and the thing will start to take off. If you stop, or other active members stop the community might die.