Ok so I have the amulet of Mara and have successfully married ysolda but now I would like to marry someone else (eg. Lydia) but no such prompts are available, even whilst wearing the amulet and having completed necessary quests. Help much appreciated.

It isn't a bug.

You can only marry once. You can't have multiple marriage partners.

You can't divorce either.

Ninja'd

How is this a bug? The only race/civilization that outright has multiple marriage partners actually shown are the Orc strongholds and even then they might not be all like that. Everyone else seems to have a marriage system similar to the West.