Hi Donna,
I checked the calendar and it appears
Fantasmic! is scheduled for 9 p.m. every night during your stay and again at 10:30 p.m. on May 11, May 13 and May 17. I'm not sure why you haven't been able to make Fantasmic! dinner reservations. However, you should still be able to see the show, even without the reserved seating section vouchers you get with the dinner package.
I hate to admit this as a Disney regular for 15 years, but I saw Fantasmic! for the first time only a couple of nights ago. We had originally booked the dinner package at Hollywood & Vine, but needed to change plans. So instead, we arrived at the amphitheater a little more than an hour before the show and found fabulous seats right in the center.
People were still filtering in a half hour before showtime.
The park was crowded that day because it was a holiday weekend. We were told by a cast member that seating normally isn't a problem as long as you get there about 45 minutes in advance. The dinner package vouchers require you to be there a half hour in advance.
We loved the show and can't believe we waited so long to see it. The pre-show entertainment and the snacks from the concession stand helped pass the wait time quickly. Plus it was nice to sit down and relax as the sun set after a long day in the park.
I hope that helps, and let me know if I can answer any more questions. Enjoy your visit!