This situation is usually caused (unintentionally) when the city in question already has shields in it's production box (i.e., is building something) and some of the those shields are "spoiled" -- they were rush-bought, forest was choped nearby, units were disbanded for sheilds, etc. With any "spoiled" sields in the box, production can't be switched to a wonder or small wonder.
BTW, if the above explain you're troubles you could: (1) since US is not continent or otherwise location sensitive, you might consider building it in a different city (with no spoiled sheilds); or (2) switch the city's production to a unit or improvement tha will be completed next turn (perhaps with a little rush-buying, and then use your leader to rush US when the production box is once again empty (and US should be an available build).
Catt