MAB if you want to make them flat e.g thinner and more dense/crisp try
to get rid of some of the raising agents that you use i.e instead of
using self-raising flour use plain flour or even a mix if you use
bi-carb just take it out of the recipe this will not change the flavour
of the biscuit it will just change the texture.

please do nnot add any extra butter or brown sugar this may change the
desired flavour but if the help abuve dous not work than try adding
more brown sugar as well as whats written above
enjoy