I just completed a one month software engineering contract. Halfway through, I sent an invoice for half (specified in the contract), and it was viewed but not acknowledged. Yesterday, the last day of the contract, I rolled it into a full invoice. The CEO apologized for being late, and said the admin side of the company was hectic since they were busy closing a funding round this week, and that he was "fixing this." However, there is currently no payment sent so I'm not sure what to make of this response and what to do next. Normally I would just be patient and wait a few business days, but the fact that the halfway payment also hasn't been sent is making me feel uneasy.
Unfortunately this kind of thing is getting worse. It's not just startups either - huge corps are outsourcing their accounts payable operations to whereever saves them money, relying on the magic of big integrated software like SAP to connect the dots ... woeful results.
With a startup, dealing directly with CEO, there isn't a fat bureaucracy to blame. They may be relying on the fundraising round to meet operating expenses. (not optimal......)
Check in periodically, ask how that fundraising is coming along. You may not be the only person the CEO owes $$ to, and they are probably stressing about it too. If you need them for a reference for future work, they might really appreciate your "flexibility" etc. in this position. Let them know you are sympathetic and willing to work with them. But don't be fooled, it might not go well (might not raise as much money as planned etc), so time to look for other opportunities.
If they get their investment, and they don't promptly pay up, that's the time to push harder.