Increase Child Maintenance request

I am currently paying child maintenance to my ex, which based on her recently stated costs, contributes 50% towards those costs (household, child activities etc..) as well as an additional £135pcm to pay upfront for additional costs such as school trips.

She has the children 8 days/nights out of 14, and they are with me 6 days/nights out of 14 (where I of course pay).

My ex has now stated that she would like me to give her more money for the children. My view (given the above) is that this isn't warranted.

Under what circumstances or reasons can child maintenance arrangements be increased?