Stop work on anything else for them right now. Send an email asking for a specific payment date, not just “processing.” If they don’t give you a real date within 48 hours, start charging late fees if your contract allows it.
For future jobs, ask for 50% upfront and the rest on delivery. Cash flow problems disappear when you get paid before you deliver the work.
Send one more email with a clear deadline for payment, like 7 days. After that, you might need to consider small claims court or a collection agency.
For next time, try asking for payment in stages. I usually do 25 percent to start, 50 percent halfway through, and 25 percent when finished. Makes these situations way less painful when you’re not waiting for the full amount.