And the intelligence of the characters in question? If it's under 20 but over 15, I'd still expect a couple of improvements in that many attempts, but ... as far as I understand the learning curves by current intelligence, it could happen, it's just a good sized chunk of bad luck.

Before lore was made easier, I'd expect no more than 2 improvements at most in 600 attempts made with any character whose intelligence is less than 20.

If your int is 23 or higher, something sounds very wrong. Even extremely bad luck shouldn't give a streak that bad with that high an int.