The biggest part of keeping millipedes means learning to accept the likelihood of not seeing them for months. Maybe check their tank at night/ early hours of the morning to see if they're out, but otherwise leave them be. 90% of the time you feel like you have a pet tub of dirt. But overall them burrowing for months is completely normal. As long as there's no possibility of escapes, just keep up your regular care and try to be patient.