Hughes 5003 lifters

i assume you've got rockers that adjust at the pushrod? if so you may not have enough lifter preload. the 'normal' 3/4 turn ish from zero lash is for stud mount rockers. that would need to be a fair bit more when adjusting at the pushrod. just a thought and it could account for your issue as the lifter piston would be closer to the circlip.
neil.