Sending a letter before action (LBA) is only as effective as the creditor’s willingness to enforce it. The risk is that once debtors know you have no intention to carry out your threat of action, your front line weapon rapidly becomes increasingly ineffective.
You may be reluctant to make a claim due to worries about offending your larger clients because you are fearful of not getting further orders. You shouldn’t be.
If you have tried every other avenue of approach with no success, it is worth remembering that a customer who is no longer paying your invoices is no longer a customer? They are a debtor.