I have not experience with running any popular twitter handles, but I can imagine that they block lots of people for various reasons, ranging from trivial to non-trivial. Honestly, if I were you, I'd simply let it go and not care if you can.

Also, from the information you have given, from a certain point of view, you might come off a bit needy: You send in 5-6 proposals per year and send direct messages and mails to multiple people when you don't receive answers and you care about whether some twitter account is blocking you or not.

I am not saying you are needy, I am just saying it could come off that way and that might have lead to either being blocked or communicated in non-tactful ways.