At the risk of repeating everyone else's questions....Why aren't tickets/reservations available on June 17th? We need to travel between Milan and Visp. Any insight would be very much appreciated

The rail timetables get updated the second Saturdays of December and June.  The June timetable updates aren't out yet.  Until they are, you won't have luck making reservations for travel after June 9th.  I'm working on June 17th tickets from Milan to Spiez and facing the same problem.  Word is that the new tables will be out by the end of May--so within the next week.  Once they are, you won't have any problem getting seats on your train.