It's pretty normal for the settlements to grow slow, this is how SS is meant to be; nevertheless, maybe it is a little bit odd that you have 0%, check settlement details and figure out what's going on there.
Look, in my current english campaign Rennes also has 0% growth (because look .... 7,5% squalor lol), but I'll quickly deal with this problem building an Abbey and the Council Chambers (the Abbey gives 5% health bonus and the Council Chambers gives 10%) so the growth will be positive again (at least +0.5%).
But most importantly ! always be sure that all of you governors have HIGH CHIVALRY, this is very important as chivalry points of the governors boost the population growth. Look what happens when I send in a high chivalry general:
Daaaaaaaaamn, 2% is something.
I hope this helped
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Also, you all know the AI is retarded so he has some advantages over the human player, including increased population growth for his settlements, but you can use this against him later in the game for example when for you it's very very hard to get a citadel, so you'll just let the AI build one (with his huge population growth bonuses) and then conquer it.