Ann: Director Changes and First Innovation and Research Centre, page-27

I too WAS in the habit of trying to help companies in which I hold shares, and like you, rarely received replies. I have come to realise that if they get back to you with a "great idea.. we will take it up" it can amount to you becoming privy to information that other 'holders don't know about ... i.e. insider trading is possible.
There must be a way to help e.g. by confidentiality agreements, but why they can't get back to you with a generic explanation is plain rude.

There's also the ego-trap. Hierarchy hates to have underlings make suggestions. The trick is to document your idea for later evidence that it was your idea, then slyly wheedle it into their mind so they think it was theirs.