Bogey plants.....

Kirengeshoma palmata especially and most woodland plants in general. Although, now the trees are getting to a decent size I might try yet again Then there's Plumeria (frangipani). I manage to keep them alive during winter, inside the house, but lose them in the Spring, usually through overwatering too soon On the bright side, bindweed, fat hen, couch grass and bryony all grow extremely well
Have you tried species or tree lupins, FC? Perhaps they'd do better than the hybrids?

 

Always wanted to grow Embothrium coccineum, had about 5 attempts over the years but they always end in failure.
Given up now.

 

@ARMANDII.....did you see the enormous clumps of Dierama growing in pure clay at Great Dixter? They were about 5' across! If you need some clay, just say the word..I'll send you some!
@pete I know what you mean .....you can only battle nature so far

 

Last year was lobelia and petunia from seed..not bothered this year, they're cheap enough!

Salvia forsakolii (longks) didn't germinate. Twice I tried, twice I failed. 2 other salvias are going great guns.
Latana camara. Not one. It did say 42 days to germination but tough tits I can't wait that long. Also, some petit grid de rennes melons and aubergines. Zilch.
Not my fault but some cat or wind took my nicotiana out so given up fags..

 

How about bunging them in pots @ARMANDII ?

 

Swings and roundabouts....the roses here are magnificent but the soil is workable for about three weeks of the year!

 

Hardy orchids - though I have a good clump of epipactis that increases every year, dactylorrhiza are only reluctantly hanging on in same area of soil, and cypripedium just look at me and die. I've spent a fortune on them over the years. Even in pots in well drained gritty soil, they get some sort of rot. Ho hum.
@noisette47 - I have found with plumeria that you just have to hold your nerve. Despite feeling sorry for them, you have to wait until the buds at the top are DEFINITELY growing before watering. And even then, give very little and let them get very dry before gving more. I have some seed grown ones that have great trunks and 2 feet tall, so hoping for flowers soon! (I'll now probably kill them this summer...)