Helebores sometimes take a while to settle in enough to flower. They need to build their stength up to flower. Mine were in a couple of years before they flowered - and they have got better and better since!
Cant see that the nettles are actualy stoping it flower but too much nitrogen will inhibit flowering. Probably needs feeding with a potassium based feed to encourage flowering, but i am not sure when to do this. Others might know.
Hope this helps.