This happens often, I park a large army in an enemy area but a siege does not occur, for many turns. In particular it happened in Flanders and Venetia. What is happening (or not happening)?
Both are ports. It will be harder to seige them unless you have Fleets blockading the port (not just outside in the sea area).
Secondly, how much is a "large army"? Having many artillery units in your army will also help increase the chances of the siege. What is the strength of the units being beseiged?
You should be reading some reports of the progress of the seige and should notice some smoke coming from the ports (showing some progress).