Like yourself, not only do I enjoy putting together game plans, it makes the game experience much better for me, since the plays I want to call in a given situation are now easily and quickly found. While it takes time and hard work, it ends up saving time in the long run.
Not surprised there isn't a lot of discussion here about both custom playbooks and game plan features. First, at this point in the cycle, most folks are busy voicing their concerns as to what is wrong with the game and what needs fixing. Happens every year and completely understandable. Also, EA touted the addition of RPO's quite heavily this year, but I heard no mention whatsoever of the stability improvements regarding custom playbooks and the fleshing out of both game planning and the play call screen itself. That did strike me as odd, since play calling is extremely important and EA took a major step here in the right direction. Finally, even folks who are interested in these features have been so frustrated over past bugs and limitations that they have simply given up and gone back to calling plays by formation.
To answer your question: First, after you save your gameplan, go back to it and make sure the plays are there as you've planned. Then, when you are calling your game plan,
I would not advise you to use the suggestion box. There is a randomness to those playcall windows, which sometimes don't always show all your plays. If you click over to the "RUN" or "Pass" play call windows, they should show up. I tested this on both the BETA and game at launch by making video clips and then going back slowly to cross-check against my gameplan. Let me know how that works out for you. Good Luck.