Regardless if it is a kit or not, you are dealing with a living organism when using yeast. Even if it is designed to finish at a predetermined specific gravity, it is always a range based off attenuation rates, temperature, fermentables, etc. If your specific gravity does not change over several successive readings, that's where you've finished and that's about all you can expect with your final gravity. Your only alternative is to pitch a different yeast with a higher attenuation into your must and hope to restart fermentation. Probably the most important question to ask is what was your starting gravity? If your starting gravity is really high, your current abv might be inhibiting further fermentation.