Why is the process of annexing vassals interrupted sometimes?

In a couple of different playthroughs I had a situation where vassal annexation stopped.

In the first case it was a single province territory (I played by France, had 100 provinces or so and wanted to vassalize I believe Frankfurt) - I could start vassalization (I had relationships higher than 190), but was not able to have more than 0% progress.

In another case (playing as Crimea, I had around 15 provinces and wanted to vassalize Georgia, they had 3 provinces) I started vassalization, earned around 50% ... and during this time completed the annexation of another vassal. As a result my relationship with Georgia became worse (dropped to 120 or so) and vassalization stopped.

I started improving relationships, pulled them back up to 160, but that didn't help to resume vassalization.

Any ideas why?

Solution 1:

In the recent version of the game annexation speed depends on your diplomatic reputation and how close your cultures and religions are. If your diplo reputation has dropped because of some event, then previously positive annex speed can drop to a halt. You have to inspect active country modifiers and look for it's expiration date. You may also read this wiki article:

http://www.eu4wiki.com/Vassal#Annexation

Solution 2:

I played earlier this year so it might have changed since then but having low relations doesn't stop the annexation. But there are some factors that could slow it down. Other than the reputation as Allen already mentioned.

Also, you need to spend diplomatic point each month. If you're out of points, it won't make any progress.