Hesitant goblins during siege?

I'm having issues with sieges.. At first, when the goblins appear, they'll all rush towards my entrance. As soon as a few goblins stumble into my cage traps, however, every other goblin in the squad simply stops. Other squads will rush to the entrance, but likewise stop after a few of them get caught. There are no doors blocking the way into my entrance, and there are even some war dogs chained just a little bit ahead of them.

The only way I've managed to get them to move is to send a dwarf near them. Once he gets within range, they'll start shooting at him then run after him. Alas, because of where the goblins have stopped, this will cause them set off some pressure plates linked to retracting bridges, rapidly sending both the dwarf and the 2-3 goblins that went after him into the floor 12 z levels below.

For this reason I can't send my military after them either, as they will all simply perish at the retracting bridge. Is there anything I can do to make the goblins cooperate, other than send useless dwarves out one by one to lure them into the pitfall, or dig a separate entrance and go at them from the other direction?

I might add that this has happend during two or three separate sieges, and it's getting mighty tiresome.