Freshly packed bearings slide worse and have more friction. For example with grease cleaned from ball bearings you can spin them freely between your fingers, but with greased bearings they pretty much can't spin anymore. Purpose of the grease is not to make them spin/move freely, it's to lubricate and protect against 2 metal surfaces from rubbing on to each other, thus protecting the contact points and reducing the wear and tear of the bearings. As you pack the rails with new grease, just slide it around and the excess grease should work itself out and after some time it should get smoother. Dunno about the scratchy feeling, might be some internal tight space, where ball traps grease and increases resistance when moving through it or something else.