Every generation is checked by the image provider before it runs. Most of those checks concern the picture being produced. Some concern the photo you supplied — the provider can refuse to work with an input image at all.
Those two look almost identical from the outside, and until now we treated them the same way. A refusal aimed at the picture being produced is worth another attempt, because the same request often passes on a later try. So the generation was retried, several times, before giving up.
That reasoning does not hold for the other case. When the provider refuses the photo you supplied, every further attempt sends the same photo and meets the same refusal. The generation was going to fail from the first moment, and all the waiting in between changed nothing.
Two things were wrong with that, and both were things you saw. The tile sat there working through attempts that could not succeed. And when it finally stopped, the message described a general problem with the generation — it never said that one of your photos was the reason, so there was nothing to act on.
Both are fixed. A refusal aimed at a photo you supplied now ends the generation at the first attempt, and the message names the supplied photo as the cause and asks you to replace it with a different one.
Nothing changes for the other kind of refusal. When the provider objects to the picture being produced rather than to your input, the generation is still retried exactly as before — that is the case where trying again genuinely helps.