"Làm ơn hãy nhắc lại" should be a correct answer.
Nhắc lại is more-so for reminding.
"Làm ơn nhắc lại" is already a correct answer. Adding "hãy" is making the sentence a little more polite, pretty suits the "làm ơn" in it.
And yes, you can add "hãy" in "làm ơn lặp lại" to make it more polite.
Couldn't you say "Xin làm ơn" too? I thought that was more polite.