If some of my elderberry jelly did not gel, can I fix it - say boil it again and add suregel (or however it's spelled).
In my experience, it can sometimes take 24 hours for jelly to set. I once thought a batch was ruined and by the next morning, it had set. If you do re boil it, I'd start with a small portion of it and if it works, then do the rest. You can always use it as a syrup type topping for ice cream, etc if all else fails.
I agree with TiggyBee, try testing with a small amount of your loose jelly and see if it tightens up. Maybe even try a simple reduction method? And like TiggyBee said, if all else fails you'll have some great syrup for pancakes, ice cream, and the like.
I agree that you should give it time to set up. Avoid Surejel or Certo, which require huge amounts of sugar and are unnecessary most of the time. If you do opt for a re-gel, why not add a cup or so of apple juice, which is high in pectin, for insurance? And be sure that it has really reached the jelly stage, either with a thermometer or by watching the sticky drops run together and sheet (one big drop from a cool spoon). A good jelly/jam cookbook is invaluable; I like Linda Ziedrich's The Joys of Jams, Jellies... because her recipes work, her advice is clear, and she avoids the Surejel/Certo fix.
Over the years, I've had ice cream, pancakes or French toast with what was supposed to be a jam, jelly or preserve of pretty nearly every fruit known to man. Even though there was a yummy result, isn't it just so disappointing? I didn't know until a couple of years ago that I could reboil the syrup for anywhere between 15 to 30 minutes and be all better.
mainecook's spoon test and/or the plate test (put a dab on a refrigerated--not frozen--saucer, wait 30 seconds; if it doesn't move when you tilt the plate, it's done) are more trustworthy than a thermometer. I do use a thermometer, though, and I begin the manuakl testing when it registers 200 degrees.
Living in Kansas where Elderberries are plentiful, I have had experience with Elderberry jelly. You will find that those pesky little berries have so little natural pectin that they are nearly impossible to get to gel without help from some kind of pectin. Apple is good because it is fairly mild in flavor and has lots of pectin naturally. But I have found that the pectin in powdered or liquid form works the best for me. I have "reboiled" jellies successfully but the best solution is to use a good canning recipe in the right proportions. Hope this helps.