Faculty, particularly ones at big schools, triage their e-mail. It is not unusual (but it is horrible) to have to wait to hear back from people. I do consider it unethical when they fail to respond, but that doesn't mean I can force them to change their behavior.
My advice is two part: 1. decide what you really want from them; 2. do not let their non-response sabotage your career goals.
In some departments there is a staff person who helps with job placement and references (academic advisor or job coordinator position), there may also be a career services department through the Office of Graduate Studies or Dean of Students Office. Try e-mailing any of those offices to get advice.
If you need references, contact each professor directly and give people short specific directions on where to submit their reference on your behalf (e-mail link, and deadline). If what you want is advice, ask for a meeting or even lookup and attend someone's office hours. But come with specific questions and requests, not for a chat.
If after all of that you still cannot get a conversation going, write-it off and look for direction and information elsewhere. If you are struggling to find post-doc or job opportunities try joining professional organizations, reaching out to former student-colleagues, or even enrolling for a class at a different college to re-establish your professional network. Try presenting a paper or poster at a conference or teaching a class at a college or community college. You need to be actively collaborating with people with similar goals to find opportunities. You need to be actively creating new things to keep people's attention (note your faculty responded to you while you were working on a publishable paper...)
This might sound strange, but instead of looking for a post-doc, have you considered applying for faculty positions? Alternately, if you are currently employed in a corporate setting, try for an adjunct position at a local college and then working your way back into a department, if that is your ultimate goal.
And this final advice is a little discipline-specific, but in some cases you could try applying for a grant and "building your own post-doc." If you e-mail a faculty member at another institution saying you are half way through applying for a grant you are likely to be awarded and would they consider "sharing" the money with you in return for it being structured as a post-doc, they'll fall all over themselves to help. (Warning: this is a lot of work) An e-mail like "NSF Grant Collaborator Needed for XYZ research from Dr.ABC" get's attention, but it's gotta be real and meaningful.