I found it I think.

There is a setting in Cura for extra width on the bottom layer. It recommends making this 50 meaning a line width of 150% on the bottom layer. I don't want extra width on the bottom layer so I have this set to 2. It appears in red adn warns you that 50 is recommended but it has always worked. For some reason all of a sudden it is making the line width 2% instead of 102% so it appears stationary, if I leave it a long long time it will eventually work right and print the item.

I have no idea why it has started doing that, it used to work fine and I haven't updated anything.