I love the look of cosmos, but have never been very successful at growing them. I've tried seed and bought seedlings, but they never grow very well. Same with morning glories and there are a few others. Do you ever have trouble growing annuals that the catalogs say are easy to grow? Which "easy" annuals do you have trouble growing?
I have had trouble with cosmos and sunflowers here in Vancouver. Not sure if it is cool/wet soil or something eating them. Are you starting seed indoors or sowing directly outdoors?
Morning Glories and sunflowers both come from continental climates where they get very hot summers; they don't like the cool, wet springs and summers of oceanic climates.
Same for zinnias, marigolds, etc. that are commonly referred to as easy in the North American nursery trade. Most of the continent has warm summers. We can usually grow these if we give them a little help getting started.
Yes I have problems with annuals too - I think I lack the knack of 'pinching'. They're never bushy enough and tend to produce solitary blooms one at a time - not impressive at all.
I have planted seeds both indoors and out. I suspected it was the climate, or maybe I was planting too early. Sometimes my seedlings get eaten before they get a chance. I've pretty much given up on sunflowers as well.
On the other hand other gardeners here do get sunflowers etc. to grow. With every plant you have to have the right site conditions.