It also helps to use the “optimize” button multiple times before adding FP+ reservations. When you optimize, you get the best plan the computer can find within a small amount of time, not necessarily the most optimized plan right away. If you hit optimize until the plan stabilizes (timing of attractions, total amount of weight time, etc. stay the same), the plan is much more likely to accept your FP+ reservations. I’ve linked the thread that talks about this technique if you’d like some more detail:
Making Optimized Personalized Touring Plans with FastPass+