My recipe for pickling beets is relatively simple, and I've never had a problem with them spoiling.

I always use new lids and make sure the mason-type jars are air tight.

While the beets are cooking (I like the small ones), I prepare a syrup made of two cups of sugar to one cup of cider vinegar, and bring it to a rolling boil. I may have to make it two or three times more, depending on the amount of beets I'm doing, but I never double the recipe.

When the beets are cooked and the skins slipped off, I cut them into halves or quarters, then pack them in warm, sterilized bottles, sprinkle with salt and pepper, add a few pickling spices to each bottle, then pour in the syrup, making sure the beets are completely covered. When cooled and labelled, I carefully, but carefully, take them to the pickle cupboard.