It is difficult to get things established in a north facing border sometimes. Make sure that the mulch doesn't come into contact with the stems and crowns of the other plants as this will make them rot off. Try using well grown plants. I have lots of ferns in shady areas and they do not seem to discourage other plants from growing so there must be another reason. The scientific name for 'inhibiting other plants growth' is 'allelopathy'. Try googling this for more info.

like you Volunteer I dont have a problem with ferns and other plants so i think the mulch may well be the problem.
Allelopathy tends to be the inhibition of seed germination rather than preventing established plants growing once planted up. I did a moodule at uni that looked at this. I got fed up of watering seeds with exract from other seeds of the same species and that of other species. enough to put you off growing seeds for life :o)

Yes, it's far more interesting growing seeds properly and having plants to plant out!

but it helps explain why some seeds need to be in individual pots or well spaced.