Did you follow the recipe exactly?
What temp is your must?
How much lemon juice did you start with?
I tried SP once and couldn’t get it to ferment either. Since then I’ve learned that partly why the recipe calls for a slurry, a healthy colony of yeast from a previous fermentation, is to improve the chances of fermentation. I also learned from
@hounddawg and others that when using dry yeast it helps to only use 32 oz of lemon juice per 6 gallons of must. Add the rest after fermentation.
If you started with more than 32 oz lemon juice, I like Dawg’s suggestion of cutting it with water, and adding sugar to bring it back up to SG 1.070 (me, I’d go to 1.085). You could try that on a small scale, say take 1/2 gal of must in a primary, add 1/2 gal water, add sugar, mix very, very well until you are sure the sugar is dissolved. Sprinkle the yeast on top to avoid any problems you might be introducing from rehydrating the yeast. Cover with a towel (don’t stir again). This will only cost you a modest amount of sugar and a package of yeast to try. If that works, start introducing similarly cut must to the active fermentation slowly, over a couple days.
My answer, when I had problems? I “converted” it to Dragon Blood by thawing 9 lbs of frozen triple-berry blend and 5 very overripe bananas, added enough sugar to bring SG to 1.090, and sprinkled EC-1118 on top. It took off immediately, and fermented perfectly.
I’m not an expert (yet), so take my suggestions with a grain of salt (uh, or sugar).