When writing articles, you should take the articles you like that make writing fun but you should also push yourself to take something in a category your not comfortable with too. Just try one a day even (if possible) taking on the same type of project over and over till it's no longer a challenge but a challenge licked.
When you do this you get an opportunity to see more information than you may have known about the subject and may actually find that you like it, but never gave it a chance. You may for example not be interested in stock trading but with enough experience in learning about the system and how it works you may find it fascinating.
This happened to me after taking as a challenge (because I knew nothing and had absolutely no interest in the topic) projects for "Forex Trading Software".
I was a bit fascinated by what I learned actually. The more you take on an undesirable topic the more you learn and can surpass the challenges of it not being interesting. That's how you conquer a hard topic and seal it in gold so next time you see it you take it because "it's easy"!
You will also find that you may end up thinking in the direction of learning to actually try stock trading- something you never thought was interesting enough to pursue or even try- but that may be your Niche-Job, something your good at and make a lot of money doing!
Your "block" may be coming from a lack of experience, not enough information, or even from something completely not related. Some things are just dull or boring or way outside your interest categories .. but you can always recruit them and change them!
Not even trying may rob you of not only the opportunity to make a once mundane or "boring" topic become something you are interested in and find higher paying work in writing (because many other ghostwriters may pass on it thus driving the price up to get "someone" to write it!