The province is under siege (the owner cannot acess the city screen etc.), but the siege cannot make progress.The sieging army isn't 1000 strong so the siege doesn't work?
Sieges never made progress if you had less men than the garisson.I've noticed that it seems, unlike past versions, that you either have the same number of attacker (sieging) men than defenders or the siege will never progress, instead of progressing with a proportional smaller speed. I could be wrong, however.