We had a paper accepted to a reputable IEEE conference.
We have completed the copyrights form and one of the authors have registered and presented the paper at the conference.
After a couple of months, the conference proceedings were uploaded to IEEE Xplore but without our paper.
Is there anything to do about this? Is it possible to check (with the PC chairs?) why it is not published?
If the paper stays left out of the proceedings, can we resubmit it to a different venue?
If you were reviewed and accepted, signed the appropriate forms, paid to attend the conference, and presented in an IEEE conference, then you should expect to be included in the proceedings.
If you aren't, then most likely it is a result of a mistake, and can be corrected by contacting the conference publications chair. The publications chair of the conference should have an IEEE publications contact, who has the right combination of authority and connections to help get it sorted out. If they agree that you should be in the proceedings, then you can treat your paper as being published at the conference even if before the fix propagates to IEEE Xplore (which can sometimes take many months). Importantly, this includes posting copies on your own site, where they can be easily accessed, per the standard IEEE copyright agreement.
On the other hand, if there is some reason that you actually did get dropped such that they are not willing to officially include your paper in the proceedings, then they are effectively relinquishing any claim on the paper and you may do with it as you wish, just as though it had been rejected in the first place.
Welcome to copyright hell! Mua-ha-ha...
More seriously, though, do the following in parallel:
I assume that just before signing the accursed "copyright form", you gave a copy to someone else, and agreed to grant him/her rights of copying, distribution, and subsequent granting of the same rights (e.g. you licensed your work to them under one of the free licenses, such as Creative Commons or GNU FDL, etc.). If you think this did not happen, try jogging your memory a bit, I'm sure it will come back to you. And I'm sure that person will later be able to attest to this having happened. Now, have that person make you a copy with the same rights you gave him. You can now publish that copy wherever you like, and IEEE can't do anything about it. Also, in practice and from (limited) experience, IEEE doesn't try to harass authors who publish a copy of their work for free online. (They might come after you if you publish it commercially somehow.)
Write:
and ask them why the proceedings have not yet been published, and to try and make that happen ASAP.
I'll add that, personally, I don't consider a "publication" on IEEE Xplore as a publication per se, because people who didn't pay the IEEE (a significant amount of) money can only get your paper via SciHub, which doesn't always have absolutely everything available, and some people are worried about the legality of getting papers through there. So it's important you also follow the parallel course of action and properly publish your paper online - make it accessible by the public.
