Garden primulas of the primrose, polyanthus and auricula types should be divided every couple of years and replanted in a different area. Not only can this outfox vine weevil (their larvae are keen on the roots of primulas), but the change of soil perks up the plants. Don't do it on a baking hot day though, as the plants may become too stressed.
Prepare the ground first (a shady or semi-shaded position is best) by digging in some garden compost or other soil conditioner. Dig up the primulas and gently pull or cut them apart at the base so that there are distinctly separate plantlets with little root systems. If no individual plants are discernible, they have failed to multiply and will probably benefit from a rejuvenating move anyway. Replant in informal groups and, if you have any bonemeal (which is no longer being manufactured for gardeners), add some to the planting hole to promote root growth.