Is the cities you're sieging at an unblockaded port? If it is, then there will not be a bigger chance to bring it down the 6th month then it was the first if it says 0%. If it's blockaded or inland the chance get a little bit bigger every month to make progress.
If it's a multilvl fort and you're sieging without artillery and/or a + siege leader it can be very hard to get it down. If the country in addition have a defensive slider, or advisors to boost defense it will take even longer.
You don't say anything about what age you play in, but high lvl forts in sea provinces is impossible to take if you don't have artillery/siege leader unless you blockade it. And even if it isn't that bad the chances can be very small. So try bring in some artillery. (I think it's 40times fort lvl, or was that EU2?), for a 3+ in siege bonus. Wich will increase you chance for progress with up to 30% point pr round.