Siege engines become dramatically more accurate the longer the bombardment has been in progress. In the beginning it is not at all unusual to have a hit rate of 10-20% for catapults. I don't have direct experience with battering rams, but I've talked to those who used them.
With regard to the rams, are you sure the city has walls? If you only have 1 or 2 rams, you could go a long while without hitting them even if it does.
By about 24 hours into the siege, usually you will be hitting 80-100% of the time.
And yes, multiple catapults can target the same building. It SHOULD go down 2 levels when that happens, but it does not always do so. (I've filed a petition regarding this problem.)
Hope you're enjoying your first siege, and congratulations on your accomplishment!