I find that when a task does not interest me, or a job, it becomes very hard to concentrate. Why?  

Not uncommon. What u r feeling is very common , however , if u have to u could convince u self this is important for u to do.
More info needed. There could be a lot of reasons for your difficulties, but you have to put in context and see for example if you actually have similar issues in other settings; it could be boredom, or a sign of something else going that you might want to a conversation with your doctor about...