How labor begins is not fully understood, if it were, less women would be induced, and inductions would be more successful.
There's an intricate balance of hormones involved between the baby and the mother. Baby releases hormones to trigger the start of labor, although what triggers baby to release the hormones is unknown. Usually it's full maturity, but that's not always true. And it's more complicated than that because if the circumstances aren't just right for labor (mom is sick, mom is stressed, even perhaps a psychological block) then labor will stall or halt entirely. And sometimes labor halts for no known reason, only to start again a day or a week later.
It's all amazing and miraculous, and ABSOLUTELY MADDENING!!! Especially as a mom in your situation. I'm trying so hard to be trusting here, but like you, it's getting harder and harder. My body seems to be so ready, everything is gearing up just right, I just can't get over that apex and go into full blown labor. YET. It WILL happen, it has to!!
I do think that I'm going to start NSTs and/or BPPs this week (my next appt is Friday, but if baby stays so quiet, I might go in tonight or tomorrow just to put my mind at ease).