Day lily not flowering

After moving into our house almost two years ago, we were given a number of plants from my parents, including a clump of day lilies.   They grow brilliantly in my parents garden in Cardiff, but this is the second summer they've been in our garden (in north London) and we haven't had any flowers since planting them.  They are in a sunny spot in our north facing garden, near some crocosmia and a spirea that both do well.  Apart from the odd hole caused by slugs in one or two leaves, they look healthy.  Can anyone offer an explanation or suggestions?

I think it is just a matter of waiting-you could try a high potash food similar to this

http://www.gardendirect.co.uk/garden-care/fertilisers/soluble-feeds/high-potash-feed

it might not produce a result this year but perhaps for next-just for the record mine have been flowering for the past week so don't give up hope yet.

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.

Thanks Sotongeoff and Robot.  I will try feeding it to and try to be patient until next year.  It does get quite a bit of sun where it is - it is in the sunniest part of the garden and other 'sun lovers' seem quite happy nearby.  I intend to divide the crocosmia next to it, as it is become quite a large clump, so maybe a bit more space and light will help it too.  When/If it flowers, it should be roughly the same colour as yours Robot.  Thanks for the photos.