This is a bit of an expansion on aeismail's answer, which already covers what I expect to be the central issue - a 6-month internship with a "junior research fellow" is just not particularly attractive to many PhD students.
Put yourself into their shoes. Your project, even if leading to a nice article, probably does not contribute strongly to their own dissertations. Most PhD students in the Western world are fully funded, so the fact that you also have funding does not help them (they would basically be temporarily replacing one funding for another). Your reputation is probably not yet strong enough to be a good experience just for name recognition and letter purposes alone. This is not to say that there are no students out there that would be interested in that, but I would guess the majority is not and finding the ones that are is not trivial.
However, I am sure you have options:
- The most obvious one is to take the funding for "a couple of students for 6 months" and arrange it in a way to fully fund one PhD student for their entire study. If necessary, try to co-fund if you have to from some other source.
- Conversely, you can try to fund undergraduate research from your own community or university rather than convincing PhD students from abroad to come to you for a few months.
- Try to work with your research collaborators to arrange some sort of exchange with them. If you have good connections to some other seniors in your field, set a collaboration up with some of them that includes a 6-month visit of one of their students at your site in the context of a larger project. This has the added benefit that a longer collaboration is surely more useful for the other senior, you, and the student than a one-shot visit and article.
- Offer (paid) internships for students from developing countries. For such students, a paid visit to a stronger, more well-known university can be incentive enough. You will need some local contacts, though, to bring you in touch with interested students and help with screening of applications.
And for more senior people, whose students I would potentially snatch up: Would you recommend a good student to do something like this? Would you be annoyed that I "borrowed" your good student, just after you taught them all the difficult parts?
I would recommend it to the student only if we already worked together, or if I otherwise had a really strong opinion of you, or you had access to something that the student really needed for their own project (say, good industrial data). I would not be interested in you "borrowing" a student if it was not clear to me what would come out of it for the student (no, "publish an article" is by itself not good enough - the student would do the same back home).