The primroses like neutral soil, but well-drained. Could it be that the soil didn't freeze well enough that warm winter and they got too wet? Winter wet is hard on plants because they are not taking up the water and the roots suffocate.

I have white and yellow primroses. I got them from a friend about fifteen years ago. Being in zone 9 I don't get many hard frosts and I do get a lot of rain (July av. 1140mm- winter). My soil is acid - it grows rhododendrons and Japanese maples very easily. I mulch with mixed leaves and old cow manure plus compost each autumn. Mine tolerate a dappled light and some sun.

What my friend did (and I do sometimes) was to break up her clumps and replant the fresh young growth - in a new location. I know this also works for Polyanthus. They last for longer when I do this, and the McMillan primiolas. I find this is more important for the coloured prims than for the white ones.

Over summer the clumps can dry back to a very miserable 'rhizome' and look near death, but they start to recover in autumn after some steady rain, similar to the deciduous primulas. If yours have faded back it might be worth digging up a clump, splitting it, and potting up the rooted portions in anticipation of the autumn refreshment.

Congratulations on keeping them going in such hard conditions.