White polystyrene bead can drop or sag over time. This type of insulation was popular in the 1980s and early 90s. Also, any works on the house, including window replacement or moving sockets can cause displacement.
The above thermal image shows a bedroom where insulation levels have dropped down from the eaves, leaving cool air to move around the empty cavity. This area can then be prone to condensation or mould should humidity levels be raised in the house.