Make sure lot control is on for the item. On our system someone turned off lot control for all items and we had to fix a bunch of stuff. The table I looked at were tdltc001, tdilc102, tdilc111 - plus tdilc401 and 402 to fix outbound and inbound. I would start there - but that is based off our system.
My latest mantra - make sure you have latest stpapi patches and the latest session object. If on LN then please explore the option of using DAL2 functionality.
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