One other possibility would be an underpitch.
@radagast did ask the question as to what the yeast was, but I didn’t see an answer (@Groucho may have missed the question).
If we assume the online kit instructions
are correct, then the yeast would have been
It appears that Morton’s Ale Yeast is a 6 gram package
It may be that the right amount of yeast was pitched (although the right amount appears to be more than what came with the kit). And there may be other reasons why the measured SG is higher than expected.
