Inspect for burrs, (machine shop should have honed the lifter bores) inspect the lifter, try a different lifter in the same bore, if a different lifter will not go all of the way down, (how far down does it go?) then try lots of prelube and work it up and down some and see if it will drop in. I have had some lifters do the same thing, and sometimes just working it a little bit will get it to go in. Do not force it, if it will not go, then maybe you need to take to block back to the machine shop for a closer inspection.