they prefer a poorer soil, so perhaps its too rich for them. they also prefer sun rather than shade. They grow well here and we are alkaline over chalk bedrock. so perhaps the soil is too acidic.

They definitely like a bit of lime on the soil.

They grow well round here with a soil just a tiny bit on the acid side of neutral. But they do really well in limestone areas.

Try mixing some limestone chippings in and around the planting hole.

They do thrive on neglect. As do many wildflowers.

Acid soil is not good for them, tho' they will do better in a lawn area, if you have one.

They are lovely.

I have acidic soil that is also moist but free draining, They are thriving and self seeding every where. they are like a weed round here in my borders. and they have cross pollinated with my other primulas too.
They need to be slightly moist for a good return. Check out the RHS site for Primula veris for the correct conditions, I'm sure they will state what soil.