Sounds to me like it's slipping on the tuner shaft... Did you go thru the hole twice before tunin' it when you last replaced it...

Might also be in the tuner itself... You won't see alot of movement fir it to go flat Maybe not enough to visually detect...

Other than those obvious possibilities, check yer bridge pin and bridge... Maybe the ball didn't get a good fit in the bridge???

Do you stretch the heck outta new strings... If not, good idea in them future...

Keep us posted...

Bobert