It is easy to forget small amounts you spend.
Like for example if you give some change to your kids, you do not always remember it, it does not go into your budget. Same when you buy a with a paper or any small items.
Try writing down everything you have spent that day at the end of the day, while you still remember, hopefully anyway:-)
Work out the total mount you have spent at the end of the week and see how you are doing! Have one section for each day of the week, try filling it out every day as it easy to forget what you have got. Keeping a diary over what money you have spent makes you become a lot more aware of what you are spending, how you are spending your money, and how your well you are keeping to your budget.
You will probably be a bit surprised by how easy it is to spend small amounts and how quickly they add up!
Do this every week for a month and see how you are doing.