Yes that's what I'm saying. You are incorrect that there is only 1 spline. When their aligned you have to look real hard to see there is 2 sets. I have rebuilt numerous sure-grip units (clutch and cone type) and know absolutely beyond the shadow of a doubt that there is 2 sets of splines, both about 1/2" wide, on each side and they have to be perfectly lined up for the axle to go in. Your description tells me it's going into the first spline but the 2nd isn't lined up so it's not going in it. Sometimes their a real bugger to get aligned once their out of alignment. Especially if the sure-grip is nice and tight.
p.s. I know some of you guys will read my post and say the cone type is not rebuildable. Technically it isn't rebuildable like a clutch type. But you can machine the sides of the cones to get more life out of them as long as the carrier and/or cones aren't totally trashed.