When you mention that "previously it use to run properly", it implies that changes were done in your system, either through updates (Support Packs/Stacks) or through configuration changes (changes to Rules or Tables done by a consultant and transported).
If the problem happens only in one environment (such as DEV), then you can compare tables and programs between environments.
If the problem happens in all environments, then look at all transports that were applied to PRD since the problem was first noticed (and since a few days/weeks earlier).
