If its customized, then you will need to check if under debug mode exactly at which point the problem is caused. You might need the technical consultants help in case you cant debug the program.
If possible put your code here. That would let people to help you.
One question, if its a lot controlled item and you are adjusting the quantitly in that lot, check if the date is the same as seen in stock point inventory.
Unless you try to do something beyond what you have already mastered, you will never grow!