IMO it's unlikely to be a nutrient problem, they usually show up once you are well into ferment and the yeast runs out of nitrogen.
The more likely scenarios are that the yeast is in poor condition or there is an inhibitory compound present in the juice: if it's unpasteurised but microbially stable, that might be because there's a preservative of some form, maybe SO2.
If you are sure there is no preservative, add some new yeast.
If it's likely that there was SO2 added, take about half of it out with peroxide. The quantity of peroxide required to do this is about one quarter of the quantity of SO2 present, eg if you had 100 mg/l SO2 in your 20 litres of juice that's 2 g total, you'd add 0.5g of peroxide which is 1.5ml of a 30% solution.
Another technique which can be used if there is an unknown inhibitory compund is to use a sacrificial yeast culture: usually best added when the juice is cold. Add the culture, mix it thoroughly then allow it to fall out and draw the juice off the cake. Warm it up and add a new culture. This works because yeast has an enormous surface area and can thus be used to "fine" out the inhibitory compound. You can use any old yeast for this, it's a good use for yeasts that are past their use by dates.
The nutrient won't hurt BTW, most of them are dried yeast hulls which can have a similar effect as above.
Edited by Lyrebird_Cycles, 12 December 2016 - 09:56 PM.