Make sure that you have "wet contact" on both surfaces being epoxied. Wet? Apply a thin layer of epoxy to both surfaces. Press both surfaces together and "mush" them together and move them slightly back and forth. Pull the pieces apart and inspect both areas to be glued. You should see evidence of contact between the pieces when you pull them apart. If the entire mating surface is not contacting the other piece apply more epoxy to the "dry" areas and repeat. Once you know that the entire surface of both pieces is "wet" and mating with the other piece - clamp the pieces in place.

I will also use some "mechanical" binding with epoxy by drilling a series of 1/16" holes (or indents)on both of the mating surfaces. When you apply epoxy you will fill the holes (or indents) with epoxy. When the epoxy cures you will have created a matrix of hard epoxy that matches the holes. I have successfully used this technique when joining the horizontal stab to the fuse or the base of the rudder to the top of the fuse / horizontal stab.