There simply are too many possibilities for us to give reasons why priests may have quite legitimately been unable to offer the sacrament (as opposed to ‘refusing to give’). The whole experience I’m sure was acutely painful and people who are already in a crisis situation may not express themselves clearly, or understand clearly what others express to them.
My advice to the OP is that, since a priest was indeed found and the sacrament was administered, that he tell the original person exactly this:
I am glad your grandmother received last rites. I am also sorry that in the process of your arranging this for you, you ran into problems. I ask that you take some time to grieve and to pray, and then do one of two things:
A (recommended): Forgive and forget. You may not have clearly communicated your wishes to the other priests; they may not have clearly communicated their reasons to you, it may all have been a big misunderstanding. There may of course have been an active ‘wrong’ on the part of one or all involved. If you are quite, quite certain that any or all of the priests were able to do this, would have and did do it for others in the same situation as your grandmother, and maliciously withheld it from your grandmother, you should immediately and tactfully let them know of your hurt, along with copying to their bishops. If you are NOT quite certainly, but you simply cannot let it go out of fear for others who might suffer the same, move on to:
B: (not recommended): Approach each priest with great humility, always giving the benefit of the doubt, going in without bias, listening carefully, and ‘repeating back’ each response to be sure you ‘get it’, and find out why they did as they did. Always be extremely charitable and respectful, document everything, and again, copy the bishop. But be aware that you will probably never get a ‘response’, or a ‘reason’, and that the time you spent on this will wind up corroding your soul. It is natural when one is hurt to want to find a scapegoat or somebody to blame, in order that the anger one feels can help ‘lessen’ the grief. But in the end, it is likely only to hurt you.
It is better to pray for the soul of the grandmother, to pray for the priests involved, and to submit to the whole affair with humility, even if one were genuinely wronged. . .and ‘offer it up’ to the Lord.
That’s my 2 cents!