You probably put too much glue. If you brushed on a continuous
coating, you'd need to clamp pretty tightly to force enough glue out
the sides to have a nice tight bond. Otherwise, you get a nice even
layer of glue that's so thick it keeps the boards apart.

If it's really going to show (and the ends of the board aren't), you
could disguise it by cutting a dado where the two boards meet and
gluing in a 36" long spline. You could even give the spline a bit of a
tapered profile, so that you can force it into the groove and have it
mash up tightly to the sides. A quick touch-up with the tablesaw
and/or jointer, and you'll never be able to tell it's two boards.

Josh