When I was Finance Director of a small limited business in the 1990's we would often encounter "cash-flow problems"

and while the cheques would be written out ready for posting at the end of the month we would have wages to pay and every quarter VAT as well.
So some of the less urgent ones would sit on my desk until shouted for. It was never personal but simply being caught between our bank overdraft limit,paying wages and taxman and waiting for "that big cheque in" our selves.
The small ones would be paid when someone "chased" up or perhaps at 90 days or when our big cheque in arrived.
So chase it up, chase it firmly, don't be put off, don't be embarrassed - it's your money.
Don't do any more work until you are paid.
If it gets ridiculous then they are not worth keeping and I personally would wait for a rainy day and stand in reception until I am paid. I have even done this, and when fobbed off, gone back within an hour and taken a placard and shown the receptionist and said politely "I know it's not your fault love, but if you don't get someone in authority down here right now I'll stand outside with this placard" (The placard said "XYZ solicitors, keeping me waiting over 90 days for payment of £225)
I had my cheque within 5 minutes! But I was prepared to lose the customer - which amazingly I didn't.