Supervisors constantly rejecting my ideas. What to do?
When I was in medical devices we had lab books to support potential patent claims etc..
Write your ideas in a lab book.
If it happens again, you could try to say something like, 'I think I may have a communication issue.  I suggested this idea on xxx but it appears I may not have communicated the concept very well.  Perhaps I'm letting myself down in communicating my ideas very well.  If it happens again could you let me know so that I can better express myself.'
You've taken it on the chin, asked them for help in a non academic way and brought to attention your original idea.  Everytime they do it, you can remind them in this manner, they'll have to take you seriously eventually; if they continue to dismiss your ideas and then suggest them later, you'll have a strong record for challenging that behaviour etc..
Very importantly be respectful, they may just find it hard to understand you.