As with any type of shoe, bowling shoes wear with use over time. With all the pressure you put on your slide foot every time you bowl, the sole will flatten, giving you more friction and making it more difficult to slide at the foul line. At best, this is a moderate inconvenience. At worst, you'll stick on the approach and find yourself face first on the lane, covered in oil.
To combat this issue, simply take a wire brush and scrape the sole of your shoe. You're trying to add some life back to the material. Don't scrape too aggressively, as all you're looking to do is take away some of the friction. Once the material is visibly fluffier than before, you're ready to bowl again.
If you don't have a wire brush, you can alternatively use other semi-abrasive items (some bowlers even use fingernails) to add some vibrance back to the shoe.
When it gets too bad and unsalvageable, it's probably time for a replacement sole or new shoes.