Most advice about disputes assumes you are the one owed money. The other half of commercial life is being on the receiving end — goods that did not arrive, work that was not what was agreed, an invoice for more than you expected.
Deal with it immediately
Raise a problem when you notice it, in writing, and be specific about what is wrong and what you want done. Silence is frequently read as acceptance, and complaining three months later about goods you kept and used is a much weaker position than complaining on the day.
Where goods are delivered short or damaged, note it at delivery. The signature on a delivery note is a statement that what arrived matched, and it is difficult to walk back.
Do not simply stop paying
This is the most common mistake and it usually costs the person making it.
Withholding payment because you are unhappy converts you from the aggrieved party into the debtor. Your supplier now has a straightforward claim for an unpaid invoice, and your complaint becomes a defence you have to establish rather than a position you hold.
Where an invoice is genuinely wrong, pay the undisputed part, say clearly and in writing what is disputed and why, and keep the discussion open. That protects your position and preserves the relationship at the same time.
Know what you agreed
Go back to what was actually signed. Purchase terms, delivery obligations, warranties, and what the supplier's own terms and conditions say — because their terms may have been incorporated into the order without either of you discussing them.
This is also the moment where the value of reading terms before signing becomes obvious. A limitation of liability clause agreed six months ago may determine everything about the conversation you are now having.
Consider what you actually need
A replacement, a repair, a credit, or your money back are different outcomes with different likelihoods. Suppliers often agree readily to a credit against future work and resist a refund, and if you intend to keep buying from them, the credit may be worth the same to you.
Ask for the thing you want rather than simply registering that you are unhappy.
When to change supplier instead
If the same problem keeps happening, the dispute is not the problem — the supplier is. Pursuing each incident individually consumes time that would be better spent on a replacement.
Sourcing an alternative before you need one is worth doing generally. A business with one supplier for something critical has no position in any disagreement with them, and both sides usually know it.
General guidance on commercial practice, not legal advice. Contract terms and consumer protection rules are fact-specific; take proper advice where the amount is significant.