In simple terms, there's very little you can do about it. Let's hypothetically say you had just successfully caught a shoplifter, called up loss prevention who then detain said party until the police arrives. All that would happen is a "criminal infringement notice" i.e a slap-on-the-wrist ~$200 fine is issued, they get carted down to the police station then pretty soon after, will simply be allowed on their way.
As serial offenders tend to predominantly be the unemployed, drug addicts or kleptos (aka lowlifes with nothing to lose if they ARE caught), they'll just keep reoffending regardless of circumstance or magnitude of penalty received. On the flip side, if you get it WRONG and incorrectly detain a suspected shoplifter who was later found to be genuinely innocent, in all likeliness you'd be sacked and lose your job over it!
Ridiculously unfair, but that's basically why staff are reluctant to report a theft, unless they are 100% certain and witnessed the theft occurring AND that the offender has left the premises, past the POS without paying for any/some of the goods.