If a store that usually offers walk-in kicks isn't available, it's likely because you've reached your walk-in limit for that store or chain. We set aside kicks for walk-ins to make sure as many Shopkickers as possible can enjoy them, so your walk-ins may be limited during a 30-day timeframe.
If you've hit the limit for walk-in kicks (great job, BTW!), the walk-in option will be unavailable. If walk-in kicks are available, the number of kicks you can earn will be shown on the store details screen.