Canning jars seal then go bad
Since you don't describe your process in detail it`s impossible to figure out the exact cause of the spoilig. Most common mistakes while canning are:

Improper sterilization of the glasses and lids.
Too short or cold pasteurization of the canned vegetables. Time and temperature required are highly dependent on the type of canned good. For raw red beets it should be 45 min at 100°C, for pre-cooked 20 min at 100°C.
Too much air remaing under the lid.
Failing to create a vacuum for sealing, allowing micro-organisms to enter the glass.