How often does Dollar Tree get new stock (movies)?

[The80s]

Do they just randomly get stuff in, like whenever, or is it set up like the big chain stores where they get their new stuff in every Tuesday? I never know when to go to the Dollar Tree so I feel like I miss a lot of good deals. As is, I usually go about once a month and sometimes I stumble upon good finds and other times it feels like there's nothing new. Am I just missing the new stock? Should I go more often? Or is it in fact totally random and there's no way to tell when they'll get new stuff in?