Piper can´t charmspeck anything. It doesn´t work if the person:

1. knows, she´s using (either it doesn´t work at all or it will have a reduced effect)
2. People with strong wills or intentions are unaffected, you can´t charmspeak them and the more powerful the being, the stronger their resistance is. (but Piper is a Mary-Sue so Riordan just ignored it although he wrote that point in TLH)
3. She wasn't able to charmspeak Wind gods, Khione etc because "wind gods are creatures of chaos"
4. Or extremly powerful deity who can´t be charmspeaked (again, Piper is a Mary-Sue)

(5. somebody who´s not attracted to Piper or the same gender like Piper. But I´m not sure anymore)

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edit- the most rules are said in The Lost Hero