The Dlookup shouldn't be in the strSQL, what is the output of strSQL?
What do you want to do with the dlookup, I expected to see someting like:
strSql = "(RoomID=" & Me.RoomID & ") And (CheckInDate<#" ...rest of the string
me.someControl = DLookup("BookingDetailsID", "tblBookingDetails", strSQL)