I think communication is key. It sucks to be out of the loop, especially if it's about doing work and not being promptly compensated or worse, at all.
mentioned, be firm. Follow up with them, but be firm in that you need to be paid for your work.
You could include in your contract some basic safety nets. Such as, ask for a 50% deposit up front before any work is started. That way you know the client is serious about the work they want to engage with you.
Also have a time period in which work has to be paid for once deliverables are submitted. Such as, invoice must be paid within 3-5 days. If not, interest is tacked on.
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