The most common causes are customers paying late, growing faster than working capital allows, holding too much stock, underpricing, and failing to provision for tax. In most cases the business is not unviable — the money simply arrives later than it goes out, and the gap widens until there is nothing left to absorb it.
Late payment is the one small businesses report most often, and it compounds: one large customer slipping a month can force you to delay your own suppliers. Rapid growth causes the opposite surprise — you fund wages, stock and materials up front for work that is invoiced later, so the busier you get the tighter cash becomes.
Other frequent causes include over-reliance on a single customer, one-off costs that were never budgeted, seasonal patterns that were not planned for, and taking too much out of the business in drawings or dividends.
Tax is a recurring one. VAT collected on sales and corporation tax on profits are not your money, and treating them as working capital creates a shortfall when the bill arrives. HMRC’s Time to Pay service exists where a bill genuinely cannot be met, and funding facilities can spread tax payments, but the underlying discipline is provisioning as you go. If you cannot identify why your cash keeps running short, that is a conversation for your accountant.