[fallen11]

rule of thumb:

if your income has not increased since the divorce judgement is made, there is no case for her to increase the child maintainence from you.
And she also has to pay the legal fees to engage a lawyer to start the case of revising the child maintainence.
Usually people wont do it so unless its very obvious that the ex husband's income has increased a lot to make it worthwhile to pay for a lawyer to engage the case to increase the child maintainence.

how much the judge assign for child maintainence not only depends on the child maintainence cost, but also the payer's income level.