There's lots of reasons your flapjacks might be crumbly and fall apart, but here are the most common reasons. Firstly make sure you bake them for long enough, if they are under-baked they will not hold together properly. Also make sure that you allow them to cool completely after baking. You can score the top of the flapjack to mark out the portions but don't cut all the way through until they are fully cool. And lastly they could fall apart if you've used jumbo or old fashioned oats, which don't bind together as well as rolled porridge oats, which are smaller in size.