How does multiple production work?

So if the city has excess production hammers and it exceeds the required the production to produce a current building, then it should be possible to produce another building? Sometimes it works for me, but sometimes when i choose to add a building B into the queue, Building A gets cancelled and put back into the city view choices and Building B is produced instead. Am i doing something wrong or are there limitations for the multiple production?

Hit shift and then click your selection to add to the end of the queue. Hit ctrl and click your selection to add to the beginning of the build queue.

When it works when you haven't used shift or ctrl, it's assuming you'll want to finish the building because you've already put some production into it.

And alt-click on a unit puts that unit in a loop.

A few things worth noting:

Setting a unit on repeat sets that unit to repeat at the end of the queue when produced.

Units that are produced with food (generally workers, settlers) will interrupt multiple production if they aren't at the top of the queue, but can be produced multiply if the preceding unit is also of the same type. For example, (Great Farmer, Worker, Settler, Worker) will all produce in one turn if you have the hammers, (Longbowman, Pikeman, Swordsman, Settler) will produce only the three military units then stop, with the Settler being the first thing on the next turn, (Settler, Longbowman, Pikeman, Swordsman) will produce all in the same turn though.

Wonders cannot be multiply produced, but normal buildings or units following them will. So (Blacksmith's Guild, Swordsman, Swordsman) can all be built in one turn, but (Swordsman, Blacksmith's Guild, Swordsman) cannot.

Also, I've run into issues where Ctrl, Shift, or Alt keypresses don't register properly due to low memory or similar. Because C2C has huge numbers of buildings, using the filter options built in to reduce the choices visible can vastly speed up adding things to the queue.