Hello Nordom, welcome to our forum.
In general, you should try not to "spam" other players with the same offer over and over again every turn. This repeating offers (when they are rejected) have slight negative impact on the relationship (attitude of the other player), therefore the chance that the player will accept the same offer next turn is every turn smaller and smaller (yet there is quite a number of other aspects which can change considerably so it could work anyway).
The best approach is to offer an annexation when there is at least a decent chance of acceptance per the tooltip and if such is rejected, offer the same in about 20 turns or so (or later)