GXP cranksets won't ever go all the way in, because they have a shoulder on the non-drive side that rests against the inside of the non-drive bearing. They do get a bit tight on the last 5-10mm on the drive side and sometimes need a hit from a rubber mallet to get it seated. Make sure you don't forget to put the splined spacer/dust cover on the non-drive side before installing the crank arm and tighten the non drive arm to finish the installation. The drive side will never be fully pushed up against the bottom bracket.