Yes, in fact it was that she was "having trouble." After she expressed frustration, I logged into the app from home (using her log in information) and found available times for Big Thunder throughout the rest of the day.
Her problem was that when she went in, the app was directing itself to look for times in the morning. That is the default search - to look for immediate times. There were no times left in the morning. When I hit the tab for afternoon, times opened up all afternoon. Not just one or two - almost any time I wanted.
We'll just have to agree to disagree about whether you were maximizing the app. The fact that I know is that on the same day you were there, I was able to get any FP I wanted (with the exception of the Mine Train), for any time I wanted, all afternoon and evening, for a party of four. It took learning the right way to finagle the app, but it wasn't too hard after that. I suppose it is within the realm of possibility that I just happened to get really lucky 16 times...
While they rode Splash at 8:50, I went in and was able to get Big Thunder for 9:00. There were also times available for 9:10, 9:40, 10:00, 10:20, etc. Some may have been cancellations that opened up. But I'm pretty sure some had been there all day, while the app was telling people "There are no FP available."
The app only changed a couple weeks ago and the new interface makes it necessary to go through a couple steps to get the true selection of options.