Try distinguishing next time whether on the phone or on the invitation: “Heavy Appetizers”. This should be obvious that the appetizers are supposed to be something significant, and not chips and nuts.
Also, if you’re hosting the gathering, and DO NOT require guests to bring something, it would be considerate to include what will be served on the invite. I don’t mean specific details of each food, but simply something like this:
‘Please Join Us for Heavy Appetizers and Drinks’ or ‘You’re Invited for Heavy Appetizers and Drinks’.
This will let the guests know if they should eat before arriving.