data not refreshing

Please try this expression instead.  It would also be better to split your datetime column to date and time columns (or just convert to date if you don't need your time component); that will help the performance of your model overall.

NewColumn =
VAR vThisID = Table[ID]
VAR vThisDateTime = Table[Datetime]
VAR vPrevDatetime =
    CALCULATE (
        MAX ( table[datetime] ),
        Table[ID] = vThisID,
        Table[Datetime] < vThisDateTime
    )
VAR result =
    CALCULATE (
        MAX ( table[Value] ),
        Table[ID] = vThisID,
        Table[Datetime] = vPrevDatetime
    )
RETURN
    IF ( ISBLANK ( vPrevDateTime ), Table[Value], result )

Pat