The Blank Page
You have a child in your class whom you know is capable and clever, but they usually struggle to make a start on their work. They sit there for inordinate amounts of time, staring at the blank page in front of them, then they do anything they can to avoid getting started. They sharpen pencils, fidget, argue, sulk, whine, chat to their friends, try to make the whole class erupt in laughter, even get out of their chair and wander around, or they put their head down on their arms and try to sleep. They probably have plenty of other self-distracting strategies too.
In this case the child is usually trying (consciously or otherwise), to avoid anxiety. They might not fully understand the instructions, or (more commonly), they are completely overwhelmed and don't think it's achievable. So they don't start.
While there is a long list of more specific reasons why a child might struggle to get started on a classroom task, the simplest one to solve is The Blank Page.
Sometimes all it takes to get that child feeling less overwhelmed, is giving them a page which is ruled down and across the middle, dividing the page into four quarters. Or if it's an art task, give them a box to draw shapes in. If it's a more complex art task like shading, then give them the shape to practise shading - then ask them to draw and shade a couple of objects themselves as well.
Sometimes it's almost as though having something on the page means the task has already been started for them. Sometimes it's just that the task is broken down into smaller chunks.
The Full Page
Sometimes the opposite problem applies, resulting in the same behaviour. You've stayed up all night putting together worksheets for your students – you even went to the trouble to think up 40 questions for them to answer during the session. But instead of excitement and praise from them when you hand it out, you have that child sulking and refusing to get started.
Let's think about what you've given them from their point of view though. It's not a blank page, but it is a very busy page. The sheer volume of questions can completely overwhelm and trigger high anxiety reactions in some children, usually because they don't want to fail at the task. So they don't start.
You didn't do anything wrong with the number of questions on the paper, because there are some young people who will love that challenge. But you might want to stop at that particular child's desk and tell them they only have to do ten questions, and any others after that will get them bonus points/kudos/bragging rights (whatever reward you see as suitable). Sometimes this child will also need you to mark on the page where they should start and stop, giving them the clear boundary they need in order to commence the task. Sometimes it works best if you choose questions at the end of the paper, giving the child more of a sense of completion because they then reach the bottom of the page.