High RSD

I trying to measure 56Fe, I am using Helium mode, and MS/MS scan type. using 5 replicas I am using a 8800 QQQ ICP-MS instrument. The RSD of the Performance report and the Autotune are below 3%. However when I am reading my samples for most of them, the RDS uses to be so high, for example 40 or 60%. If I read those samples again,  usually the RDS goes down. 

Please, could you help me to know what could be happening?

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