It’s usually because of lack of sufficient reason to be doing the thing or lack of a sense of control in the initial engagement–physically, mentally, or emotionally. If the life of someone dear to you was dependent on you finishing the writing project in the next 24 hours (an outcome meaningful enough, I would guess), we think you’d find yourself breaking through some resistances to getting started. Or, if you absolutely knew what the first four paragraphs were (control), you’d find it easy to get started.
At the deepest level, our own sense of self (self-image, self-esteem) is probably the ultimate driver or inhibitor of our actions, and that’s another seminar! But given whatever that self-image is, clarifying the value of the purpose of something and getting a clear next action about how to get started will at least give you the best ammunition, given whatever energy of confidence you start with as internal resource.