Why is it that the more core work I do, the harder it seems to get?

As you become fit it takes more work to continue to get greater gains in fitness. This is true especially for your core muscles. As your core becomes stronger it will take more work to fatigue it and get a good workout that makes you even stronger.

This most likely means that you are learning to engage the muscle groups in the core more fully. You are becoming more aware of your body, which is a very important part of building strength and balance. You may also have started to work the deeper muscles of the abdominal wall, way to go! Keep it up, abdominal muscles can begin to atrophy quickly. Work them daily, in your exercise routine and by engaging them in your daily life.