It's not 100% clear from your question whether your client is an agency (doing work for other clients) or an ordinary business whom you are freelancing directly for.
If they are an agency and the work is ultimately for someone else then this route has worked for me before...
In the UK (possibly the same in the US) in the absence of any written agreement (even if you are being paid) then all the intellectual property rights in what you create for your client remains with you.
Remind your client of this fact. And state to them that if they do not pay you, then you will be forced to contact the end-client and tell them that they are using your intellectual property without permission.
Usually just the threat of this will make your client pay up. On one occasion I was working for an agency that still didn't pay me (after I used this threat) so I did contact the CEO of the end-client, I was paid within 24hrs and the end-client fired the agency I was working for.
If they are an agency and the work is ultimately for someone else then this route has worked for me before...
In the UK (possibly the same in the US) in the absence of any written agreement (even if you are being paid) then all the intellectual property rights in what you create for your client remains with you.
Remind your client of this fact. And state to them that if they do not pay you, then you will be forced to contact the end-client and tell them that they are using your intellectual property without permission.
Usually just the threat of this will make your client pay up. On one occasion I was working for an agency that still didn't pay me (after I used this threat) so I did contact the CEO of the end-client, I was paid within 24hrs and the end-client fired the agency I was working for.
It might be worth a try.