ok blueless there are a couple of things this could be. with almost all polyurethanes bubbling can ocur if the can itself has been shaken, also, make sure you are brushing in long even strokes, if you spend too much time trying to smooth out your strokes as you are laying it down you will definately see bubbles, its better to have it slightly uneven because you would have to sand it any way to apply the next coat. also, wiping the surface down with 100% mineral spirits or paint thinner that contains 100% mineral spirits after sanding will 1. clean the floor without leaving a residue that will harm the next coat and 2. remove all of the sanding dust wich if left on may or may not make little divets in the final coat. as far as how to fix this? its going to need to be sanded and a new coat is going to have to be layed down- you should have a smooth floor and thats the long and short of it, polyurethanes are not going to be as smooth as gass but they should be pretty damn close