If I get blocked, it is generally because I don't know enough about some aspect of the story or the characters. The answer for this is generally more research, or making more background notes, so the place and person can be more fully realized inside my own mind.
Actually, after while, finding the ideas is the easy part. Sorting them through and turning them into stories, now, that's the hard work.
I'm mostly a novelist these days, but I have written short stories in Fantasy, Science Fiction and horror.
First and foremost, The Quiet Invasion is a first contact story. What would we do if we actually found evidence of alien life out there? It's also about politics.
As far as the writing goes, I started telling stories as soon as I could talk, and started writing them down as soon as I could string words together.