Probably incidence of the floats. With the tops of the floats level, the wing of an ugly stik should be a little (1/2-3/4") higher in the front than at the trailing edge.
Some newbies put the noses of the floats too high so that the floats will pop up on top of the water. They will, but then the wing can't get any air under it to lift you off. See the Frequently Asked Questions in my website. Send pictures of your plane, so we can see what's going on...