The key with singing is the breathing - breathe . . .
I don't know if there is a single note that gives me trouble - but keeping my breathing regular and allowing my voice to be carried by my breath. For example, in the trio I am in, I am taking the high harmony on I Know You Rider and my first note is the "A" at the second fret of the G string and then go up from there; by the time we get to the coda/closing chorus, I am out of breathe and thus can't necessarily hit the higher notes. On the other hand, by concentrating on breathing, I have found that I can actually sing higher up the scale then previously thought.
As far as hitting notes, my trio will actually take and sing scales to each other - for example A B C# D# E F# G# A. Then we will do three part harmony on A so that I will start at A, one of us will start at C# and one will start E. I don't know if any of this helps, but I have noticed my singing improving and more importantly I sing with more confidence now.