Having some issues with an array
Your issue is a C programming issue and has nothing to do with the Arduino specifically.  You should research how to implement a circular buffer.  You don't want to move around the values within the array, but rather just increment array index values.  Keep two index values, one of the current index position (the place where the next recorded value will be placed) and another of the number of values recorded.  Eventually the number of values recorded will be equal to your maximum array size and only the index position will increment.  When the array index exceeds the maximum array size it wraps back to 0.