There is always a jar that doesn't seal, its just part of canning, its called the old maid. With citrus, pressure is not required, but the water bath should last 30 mins.
You boil the jars totally immersed in a pan of water with a cap of Clorox. You can't taste it and it insures sterility and it can't hurt you. Boil the lids also. Remove jars from boiling water and don't touch again. Fill the jar to 1/4 inch full with whatever, put on a good cap, not all cap/lids are good even new, inspect the rubber sealing surface for pits, cuts or gouges. Then add the screw ring. but do not take down tight, Boil 30 mins., remove and tighten. Let set 24 hours and nearly all will pop. The ones that don't can be redone again from scratch or I just eat them. If you do 4 jars or 400 jars you'll always get old maids.
The very best thing you can do if you plan to can is acquire a "Ball's Blue Book", its the canning bible. It also has all of the recipes your great grandma used when canning.
As to your specific question, a properly sealed jar does not need the screw seal. once the lid is pressure sealed it should take a bottle opener to get it off. Its why all the little old ladies only bought lids, and why the lids are still sold separately. One screw seal could be used on loads of jars.
