Then you need to let them.know that. Make them aware that you're unhappy, and that you're such a value to them. They'll connect the dots and realise that they need to satiate your demands.
If they think they can get away with underpaying you then they will. It's easier to hire full time, so they'd rather keep.you at part time as they know you will, and will do extra hours when needed. They currently have no reason to give you what you want.