Hi. I actually don't have an explanation, and not sure why it's happening.
Each card has an "interval". When studying, you'll see the card's current interval, and the next interval if you answer wrong/correct/strong-correct. The target interval is 30 days, by default. So progress is just a % of all cards' intervals towards this 30 days goal.
If 50% of cards have interval of 0 and 50% have interval of 30+ days, progress is 50%.
If all cards have interval of 15 days, progress is also 50%
If you are repeating missed cards (quick repeat before full interval is up), the interval generally doesn't increase after a correct answer. In this case, I can see how the progress might stay the same, even though you answer correct.
If you change "cards to study" and include more cards, or add new cards to the deck, the progress % will likely go down.
But otherwise, I don't know.