If you’re using a nested repeater then your second repeater loop needs to be completely within your first repeater loop
if (have_rows('repeater')) {
while (have_rows('repeater')) {
// show field from first repeater
// nested repeater
if (have_rows('nested_repeater')) {
while (have_rows('nested_repeater')) {
// show fields from nested repeater
} // end while nested repeater
} // end if nested repeater
} // end while repeater
} // end if repeater
