Take a deep breath and relax a little. Transitioning to being a nurse and orienting to a unit is difficult.
I am orienting to the OR after a year of neuro med surg tele. When I oriented to my unit I worked on for most of the past year - it was rocky at best. The facility made new RNs orient 40 hours a week on day shift only. Over my orientation, I oriented with about 10 nurses on this unit. Was it frustrating? Sure it was. But the fact of the matter is - there are a LOT of things each person you are with has to offer. Each person had something they were super good with. Know what? I learned something from all of them.
One of my coworkers, one of our charge RNs explained why she always nitpicked - she wanted everyone who oriented to and worked on our unit to be "good enough for me and/or my family." A few months after I went off orientation, one of her family members was admitted to the hospital. She called the transfer center/bed board and demanded placement on our unit. She called the unit and asked us to make sure we held a tele bed for her family member. Her family member spent over a week on our unit. Think about that. It kind of sucked ping ponging around from preceptor to preceptor. But I learned so much. All of my coworkers learned a little bit about me and how I handled situations.
It's going to feel rough for a while yet. A skill we have to use as RNs is self assessment. Assess your own progress, what are you getting better at, what are you needing work on? Maybe it will help preceptor B see more growth if you take more intiative/become more assertive about your orientation and learning. Make yourself a brain sheet / checklist to help organize your routine and thoughts and documentation. Other than that, it's a ride and it will get better. Difficult days on orientation or difficult situations help prepare you for reality on your own. I think if you keep at it, you'll do awesome.
Previous posters are right - one probably just expects more. It's easy to clear the bar and do the very minimum, but why not become super super super at your job? I worked with nurses who did the minimum and nurses who were awesome and knew a lot and were great resources. People could complain about getting patients that 'so and so' had because they didn't do a great job. I didn't want to be talked about like that and I wanted to be super super super at my job. And I would rather hear criticism from someone directly than as gossip later. It's possible your preceptor B wants to save you time and hassle of learning something the hard way later.
Best wishes to you, it really will get better