This is totally a resources issue.
It can’t be entirely that. In each of these cases, the CPS felt that they had enough evidence for there to be a reasonable chance of a prosecution, meaning that they felt the accused would be more likely than not convicted based on what they would present. In one case was there actually was a conviction.
In other words, evidence that would probably be sufficient to secure a conviction existed in cases where it now seems almost certain that no crime occurred. That suggests to me that there is an issue besides under-resourcing.