We are sleep training our 5.5 month old this week. We're doing what most closely resembles Ferber (I didn't read the book, but we are laying down when awake, no props, intervalled check-ins (our check-ins are mostly verbal - sometimes we'll pat for 30 seconds but that seems to ramp her up)).
We've successfully removed nursing from bedtime, and I think we have a solid routine.
I've Googled "why sleep training is failing", and I feel like we have covered all of the pointers (consistent schedule, good routine, no props, laying baby down when awake, noise machine, dark room, etc.).
We're on day 4, and it's HORRIBLE! She's crying for 15-30 min before naps, generally just very unhappy during the day (she's usually such a happy baby), and at night, she's crying for 45 min, sleeping 45 min, and waking up again to cry more. As soon as we walk into the room to start the routine (sleep sack and song), she starts crying. She used to be okay being left to play for a bit on the floor (during the day if I needed to do a quick chore), and now she screams as soon as I sit her down. She's associating our room and being put down with being left alone.
The first few days were hard, but it seemed to get progressively better. Yesterday and today have been so, so hard. I need encouragement. I'm feeling really upset by hearing her cry.