Neck honed to .336 yields approx. .3365"-.3375" OD neck depending on brass thickness.
lapua .308 brass necks avg .0145"
Avg OD neck .3370" - (.0145x2) = .308" which depending on your bullet may or may not be enough neck tension to hold your bullet. Coupled with some properly annealed brass and some not annealed quite enough, differeing brass neck thickness, you will see some bullets dropping through the neck.
Your lee die is made to size smaller and then the expander ball will open the neck up so the neck isn't too small but enough to maintain about .001" neck tension per the factory.
My guess is your neck is probably honed out a little too much. I'd try for a .334-.335" honed neck depending on whether or not your turning brass and what your brass neck thickness is.