I think the problem here is sunshine and I'll try to illustrate it with photos if I may without sounding like a flipping know-all. It is just that I notice different flowering patterns with my own day lilies.
You say you have a north facing garden and DLs do need a lot of sun. I have various sizes of clumps in the garden and most do well without any attention like these in an open sunny site -
but notice how these lean towards the sun as it comes around the corner of the house at about mid-day. Also this lot are in very poor soil -
but some are behind high shrubs and don't see the sun until late afternoon and are like these - the one at the back hasn't flowered at all in 3 years and gets the least sun but I haven't got around to moving it yet and the other two are this year's bits from the larger clumps, but they have flowered sparcely.
On the other hand - perhaps you have the smaller variety (sorry don't know name) which prefers a moist soil. I have two such little 'uns in my bog garden and they are just coming out now. They get the sun for most of the day and the soil is rich and always moist.
Perhaps if you look at how many hours of sunshine your parents' plants get and then compare to yours there could be the answer. I believe that crocosmia and spirea will do well almost anywhere.
Finally - and sorry to bang on so long - it may be that the clump needs to be divided but not this year - do it early spring after the frosts or else the plants will be damaged. I only dig up bits off the perimeter of my bigger clumps and dot them around - hence they are all the same colour.
Quality of photos not too good as we are very overcast today.