Developers tend to think the solution they built is the "right" solution and don't like the concept of reinvesting their time to fix something that isn't broke.

We experienced the same inflexibility even with the paper prototypes which are so easy to fix! Hopefully making people aware that changes now will save more rework later can have a big impact. In addition, watching a real user get confused and frustrated with a paper design can cause developers to recognize the need for change.

A word of advice, that we all need to follow; avoid the temptation to interrupt an user with "but what we are trying to do." or "you don't understand the model." If they don't understand it without you interpreting for them, it is a sure bet that an user sitting by themselves out on the internet will have the same problem.