Primula seedlings are very sensitive to concentrations of mineral salts found in fertilisers. If the mixture is too strong the seedlings may not grow at all, - or the young roots may be killed. Always use a seed compost and look for one which is as gritty and fibrous as possible. Primula roots need air to grow and develop. If the compost is too fine, a few waterings will drive all the air out.
Leave the lot in a shady place outdoors. Inspect regularly and water as necessary. Do not allow the compost to dry out. As soon as the seeds sprout remove the cover. Protect against slugs. Polyanthus and primrose sprout within three weeks. #directsow in pots March 15th, covered until #germination
Primulas establish much faster if a good root system has developed before they are moved. Wait until you have at least four leaves before you prick out. A feed with a half-strength high potash fertiliser a week before pricking out helps.