Float is hanging up on something or the windings on the reostat have given up the fight. Pull it out and check it against the gauge. Does not have to be in the tank to work. wire it as if it were in the tank and move it by hand.
You didn't mention if you had changed the sending unit too... Could be your old unit is rusty or you didn't get the ground wire attached properly. Improper ground is about 80% of the problems with sending units.
I your car is older than 1965, check the sending unit by using a ohm meter. Pull it out of the tank and hook up a ohm meter to it . Empty should be 0 ohm and Full should be 30 ohms. My be in the sending unit.
Sometimes you can bend the float arm installing them.
or if you have it installed with a twist off to the side the float arm can hang on the side of the tank.
Forgive me but never had one out of our year cars so this is just general info.,,dq