The idea for this post came from seeing similar patterns with my beginner learners. All these things are so common and so simple to fix, so I thought a checklist might help:
Have I got the right sort of paper?
If you are working with watercolour you will need some watercolour paper, for pastels you will need pastel paper etc. If you are unsure I give some advice on my Art Materials page.
Have you got enough materials?
It sounds basic but if you use a lot of paint, make sure you have all the key colours you need. The art store could be out of that colour when you most need it.
Have you set up the still life, subject in the way you want it?
Sometimes we forget we have choices, move the objects and create a composition you are happy with. Is it well lit? Are you happy with the angles? Do you need that jacket that you could be painting for the next six weeks? Is the arrangement in a convenient place if you need to leave it and come back?
Have you got the right size paper?
Will your drawing or painting fit on the page? If not you may need to work larger.
Have you taken a good look at the subject?
Before even starting anything, take a good look at the subject. Identify the shapes, colours and textures. Look at the positive and negative space. You might start thinking about which brushes and techniques would be best to capture the subject.
Have you started mapping out the drawing?
This is key to working out if the piece will fit on the page, getting a basic outline of the whole arrangement before starting.
Have you mixed your colours, tested out your media?
For example if you are working on a portrait, you might like to mix a range of flesh tones before you start painting. Try out the media in your sketchpad before you start creating art with it, see how it behaves and the colours it produces. Do some mark-making and experiment if you need to.
Are you inspired by the subject matter?
If not you might want to have a rethink. Is it too challenging? Maybe you would be better with one small object to start with?