I have been in the same position as you before. I think it’s because, after losing a fair amount, the body goes into starvation mode and adapts to the ingestion of fewer calories.
I have a theory - for which I’m not aware of any scientific basis - that, after losing two or three stones, it might be a good idea to stay at the same weight for some months to allow it to adjust again before starting to diet again.
I accept that this might be complete nonsense but when I reach two stones off (I’m halfway there) I’m going to give it a try.