Stuck fermentation, help!

I am fermenting a new beer with champagne yeast. My starting gravity was 1.040 and I am aiming to finish at 1.013 but it has been stuck at 1.020 for 4 days now. I have tried adding extra yeast but still no change in the gravity.

I think the problem is the initial mash temp was 68-70 deg C (I was aiming for 65 deg C) and I think that I have produced too many long chain, unfermentable sugars.

I want the strength to be 3.5% abv, what can I do to achieve this. I was thinking about adding some sugar, but I don't know how much sugar to add and I don't know if this will even work.

Please help!!!!

when you say you've added more yeast, was it more champagne yeast? the wine yeasts dont do so well with complex sugars, so that coupled with a high mash is why you are there. try adding some ale yeast, should get you a bit lower. if you just care about getting to 3.5%, you can add 2.5oz of sugar per gallon to get you there.

Right, I have solved the problem and have now reached desired gravity. Here is what I did:

First I assessed the viability of the yeast. I filled a container with a litre of warm (20°C) water and then mixed in two tablespoons of granulated cane sugar. I then skimmed some yeast from the top of the FV and took some from the bottom of the FV and added it to the warm sugar water and left it for 15 mins. After 15 mins, the mixture was bubbling and therefore the yeast was still viable.

Knowing the yeast was still viable, I decided to try a yeast nutrient. I added Tronozymol (a wine/champagne yeast nutrient and energizer) at a rate of 1g per Litre, directly into the beer. I then stirred the beer with a sterilised paddle.

After just two hours the size of the yeast mat on on top of the beer had almost doubled and the obvious signs of fermentation were there (bubbling, and a strong smell of CO2).

Two days later the Fermentation was complete and the desired gravity of 1.013 had been reached!!