This issue occurs in v30.0 and above of Sage 50 Accounts. If you add a Net value discount with a higher value to any of the invoice item lines, the invoice will not update to ledgers.
UPDATED: This issue is resolved in v30.1 and above of Sage 50 Accounts. If you aren't using the latest version upgrade now > If you are unable to upgrade currently follow the workaround below.
The workaround for this problem is to apply the discount to each individual item line rather than the total invoice.