What the problems with my system weighing ? Help me !

I have a weighing system of BCS Italy, with 4 loadcell 15T and the load usually ~30T.
I have problems with it. When the load come to the vessel, the weighing indicator value OK, but when load stay at vessel in 1~2 minutes, the value is decrease, one time 10kg, maybe decrease 50kg in 5 minutes. And when the load move out, indicator not Zero, it negative (maybe -30~40kg). After 3-4 minutes, the value increase to Zero. I try very method but I don't know what the problems, can you help me ? (sorry my english not good, i'm from Viet Nam)
Can you tell us a little more about the type of load cell you are using;
also how your vessel is mounted to be weighed.

If I understand your point correctly - weighing is initially correct
but suffers short term drift until the load is removed.