What I gave trouble with? Well, maybe romance but that is mostly because I am a very unromantic person and no amount of romantic music can change that. I often takes the route by writing people and situations first, then I go oh, these two have really good chemistry, let's try a romance. With other words I rarely write a story with romance in mind but add it in if I notice that there is an opening for it.
And sex scenes… I am just plain horrible. Going in like, "then they had sex." Often I just skip the part. I am the personification of fade to black. Well, it has become a point of mine to write out sex scenes if only because I often write BxB. I started because I wanted to write about the romance between men as all that I have read focus on the sex part. It became more like a porno than an actual romance.
I don't really have a problem with worldbuilding as that is my strongest suit and as an advice, write the world before you start your book. That makes it easier. Especially characters. If you just go on fantasy or inspiration, whatever you call it, then you will encounter the infamous writer block sooner or later. I have never encountered it myself but I know several people who has a hard time with it, most of the time its because they haven't really planned and structured the story in a good way.
A good thing to always ask yourself questions, write out how the character's personality is then ask yourself why? Is it genes? from who? If not, then what happen to make the character this way? Create a picture of the characters childhood and ask yourself how this would affect a person growing up.
You are writing a murder, all the clues are fine but ask yourself why did the murderer do it? Why is everyone reacting the way they do.
Writing about a war, why is there a war? Why are the character fighting it? How has it changed him/her?
Writing a romance, Why did they fall in love? Is it because they are the same? They are different? Do they change each other? How? Should they stay together? If yes, or no, why?
Asking questions also works great to get rid of writer's block. It helps you lose up the knots and start writing again. Back away for a while and look at the whole story, then you might see what your story is lacking.
These are some of my advice, sorry for making it so long 