I am new to canning. Last week I made a yellow tomato jam. It turned out wonderfully. Yesterday I made another batch of it, but it didn't set (gel) as well. Is it safe to pull off the lids, recook and then reprocess? The jars sealed well, but the contents are just runnier than I wanted.

I read on another website that I could put 2 TBSP of lemon juice (per quart) into my jam and bring to a boil.

Any ideas on whether this is safe or effective? Thanks.