Just to play devil's advocate here though... sometimes it really is that "the mentors are the problem". In environments like this there is often at least one strong-willed individual who knows everything and can do no wrong. (C'mon, you all know who I'm talking about.*)
Mentors are human; humans are imperfect. It might not be a bad idea to reinforce and emphasize that Gracious Professionalism applies to everyone, not just the students! Engineers are often in this career because they are extremely intelligent, driven, and with that sometimes comes a big ego. And it is human nature, nobody likes to think they are wrong. And with something like FRC when you become passionate about an idea or design it becomes an extension of your own identity, e.g. telling me that you don't like my idea is seen as a personal attack on me.
At least engineers are (or should be) open to quantitative analysis ("if you can PROVE me wrong, I will accept it") but even then, some people just have hard heads!
If this is the case and one or more of your mentors seem to be unreasonably stubborn, then it may be time to have a conversation about what FIRST is about, who should be driving the decisions, letting kids make their own mistakes, etc. It's not a fun conversation to have but it will help the mentors with their own personal development as well.
It's not just engineers and FRC; I volunteer with another non-profit organization and deal with this all the time. And, oh yeah, at my workplace too.
*I know, I know... I'm that guy sometimes. We all do it. The trick is recognizing and acknowledging it and being intentionally open minded.