Something that always forcefully reminds me of the lack of communication between researchers and archivists is being apprised of some research proposal - in more than one instance, a research proposal that has gained funding - where the researchers have done a lot of homework and put a lot of thought and effort into constructing their proposal, in some cases have even already invested in intercontinental travel -
But at no point have they found it necessary or desirable to contact the archivist i/c the material they intend to use in the project.
Which - and again, this is more than a unique experience - is not actually available for research, either because it has not yet been processed (and I don't think it is entirely breaking news that pretty well all archive repositories have a cataloguing backlog, even if these aren't as monster as the secret Foreign Office archive lately in the news) or because there are other issues such as Data Protection or the wishes of the donor affecting access. In one case, though the acquisition of an individual's papers had been announced, actual physical transfer to the repository in question had not in fact occurred.
A catalogued archive has robust referencing that means it can be effectively cited (though another of my archival moans is the failure of researchers to use the unique identifying references archivists have created) and one that it's possible to find things in without starting at box 1 and sorting through until reaching the last box - a process that is not considered best practice from the point of view of long-term physical preservation.
If you let the archivist/s know in advance that you are interested in putting forward a research proposal that depends on a specific archive, this will feed into their management decisions about cataloguing priorities and they may, in fact, get that archive processed. Whereas it is fairly unlikely that they will be able to just drop everything and process a collection because a researcher has turned up wanting to consult it.
I also consider that funding bodies could be a bit more au fait with this issue and spend some modicum of time when assessing proposals to ensure that the archives in question a) actually exist and b) are available to researchers.