You mentioned a burrow. Are all your dwarves assigned to burrows? If so, this almost certainly prevented the task from completing.
Also, I recall hearing somewhere that jobs are often completed in order of their location from top left to bottom right. Maybe that was related?
They were. One of my original attempts to get the task to complete was to assign all the dwarves with necessary skills to a burrow, put stockpiles containing everything necessary in that burrow, give them all living quarters and amenities in that burrow, and then wait. All the other magma-related workshops and furnaces got done nearly immediately, but the magma forges for some reason didn't.
When the magma forges did complete, I had unassigned from the burrows and done away with that burrow, although I'm probably going to reestablish it with the master class artisans whose quarters are assigned down there, to make sure they actually use the workshops there.
I actually think my problem was simply the fact that I stupidly decided to furnish the dining hall with silver tables, which were ridiculously heavy, and the weird, convoluted path through mostly useless exploratory mining passages caused this to take an absurd amount of time (8 months or so), and that once I completed a grand stairway (3x3 up/down stairs for well over 100 levels), all the tasks bottlenecking everything cleared up.
Also there was a goblin siege in the middle of this and the scramble to the safe burrow caused everyone to drop the tables and everything else they were doing while we fought it off.
It's funny to think I may have nearly lost a fortress because of an error in hauling furniture.