I am only an advisor to the client, but I am aware of their difficulties in recruiting suitably skilled staff. When doing any recruitment, whenever you can't find the "right" candidate, you always need to ask if you are doing something fundamentally wrong, if you need to tighten or relax the criteria.
I know that the client was looking to fill a contract vacancy last year, and they utilised a recruitment agency to advertise the position and pre-screen the candidates. Sixteen people passed these initial tests. In a nonexistent ideal world, every one of those candidates would be perfect for the role. But that is only ideal for the employer. It is not necessarily ideal for a society to be in a situation where fifteen candidates are all equally capable of the job, invest equal time and effort and are equally disappointed by the outcome.
But as a practical target, if you are looking to judge if your criteria were too strict or too lax, if about half of the applications are rejected at each stage of the process, then you shouldn't be going too far wrong.
Let us look at historical (hypothetical) example. Out of those sixteen pre-screened candidate CVs, eight were considered suitable for interview. Of those eight, after initial phone discussion about the logistics of the position, four actually turned up for interview. Of those four, two were offered the position. Of those two, one accepted it.
So I conclude that, in this case, the client was doing things about right. I only wish they would do things more quickly.