Agree with the above: you didn't do anything wrong, the system has just been changed to close the stacking "loophole."
If your goal is to get as many Lightning Lanes as possible for the attractions with the highest lines, one effective strategy (so long as you're not visiting during a major holiday or vacation week) is to book experiences that are available the soonest, BUT also bearing in mind which attractions' return times get pushed out the fastest, which should take priority, especially if you can get a return time within the next 2 hours, or close to it. Then, keep refreshing whenever you have any downtime or are waiting in a queue, to see if you can move any of your already-booked return time(s) to even earlier. (*NOTE:
Modifying a LL in the app doesn't reset the 2-hour wait time that would otherwise apply.
Canceling an LL and making a new one, however, does, so don't do that!) So long as you avoid scheduling anything more than 2 hours out, or are able to modify LLs that are more than 2 hours out to bring them within the 2-hour time frame (exception: Slinky at 7am, which you should take whenever you can get it), you can choose a new LL every time you tap in to your most recent one, considering the same factors.
It's a complete and total PITA, and requires you to be constantly on your phone and aware of, and balancing, each ride's individual "push out" speed and the availability of quick return times, but if you want to make the most of the system, that's the not-so-magical deal.
This article (linked below) may be helpful in providing the background knowledge necessary to make those choices. Incidentally, we followed the same rankings the article prescribes when we used G+ for HS last August (with our chosen attractions/G+ being Slinky Dog Dash, MMRR, Rock 'n' Rollercoaster, TSMM, Millennium Falcon, and Star Tours, in that order: we booked, repeatedly modified, and used G+ for all of them, plus had a TS meal, did Muppets and Rise of the Resistance, got some snacks and browsed some shops, all before mid-afternoon).
https://touringplans.com/blog/genie-priorities-at-hollywood-studios/
*FWIW, in my opinion the very best and most efficient way to do HS, if you have parkhoppers AND multi-park G+, is:
don't even bother going to HS until the evening, when there's a noticeable dip in crowds. Spend your morning at another park (e.g., AK, where you can do everything with short waits, without G+, so long as you arrive before rope drop and tackle things in a sensible order, starting with Flight of Passage, Na'vi River Journey, and Kilimanjaro Safari). Make your 7am G+ for HS (e.g., Slinky Dog Dash), and then 2 hours after HS opens (e.g. 10:30am) and every 2 hours thereafter, continue to make G+ reservations at HS, modifying them as necessary to ensure that the return times fit with your anticipated arrival. By, say, 4:30pm (if HS opened at 8:30am), you should have your top 5 G+ selections booked at HS. Hop to HS around that time - if you haven't done so already - and start using your G+. If you want to do Rise or if there's some other must-do for which you couldn't get a G+, save that for last, getting in line just before the park closes. Having knocked out all the top HS attractions, you can come back another day for a leisurely "clean-up" visit and tackle the attractions with shorter lines, live shows, etc.
