One of the things which children find quite hard to do is to set out a question in the best format. In the Key Stage 2 tests addition problems are rarely, if ever, set out in the standard form that children learn.
This page tries to help with the addition of two 3-digit numbers. The first 10 questions are set out in the standard way, but the next ten are written across the page. When faced with questions like this it is important to re-write the sum to have the best chance of success. Of course, it is important to keep the numbers in line (units under units, tens under tens etc) and squares have been provided to help with this.
A very useful page for those who are still uncertain about how to approach these questions.