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Not dark enough, i.e. there is still a very small amount of light that can show around the sides because the sealant between the u channel and the wall is not absorbing it all.
Personally I don't understand why the 90% reduction in light from daylight blinds despite the leakage at the sides is not enough, I don't really get why people want 100% darkness during daylight hours. Seems to be a new requirement. Each to their own though.
As you are asking about the practical problems achieving that though, in my experience people are rarely satisfied with the results and don't achieve 100% blackout without a lot of work.
Airhead-
I pretty much require 100% darkness and silence to sleep, which has been a problem for a while with a snoring SO – and now we've finally moved up to somewhere with two bedrooms the house is [oh woe is me] bathed in glorious sunshine from every angle by 4am. There are worse problems to have, I'll just not sleep in summer I reckon. Stupid bay windows.
jackbepablo
Do you mean they later decide it's too dark?
The problem is our current blackout curtains don't seal against the window tightly enough and so light gets around the edges.