It's much too easy to become trapped in an endless loop of immobilizations (pins) during a grappling fight. While pinned, you can do nothing except try to break the pin. If you succeed, your opponent is immediately free to establish a new pin. Until they fail on the roll to immobilize you, you're effectively disabled. This isn't fun and isn't realistic. The former we can fix. (The core grappling rules are geared for cinematic struggles instead of Olympic bouts, so realism is optional.)
When the enemy escapes from a pin, their body is free to move and you don't have the same leverage as before. If you immediately try to pin them again, you haven't had time to establish new, proper leverage. After immobilizing a target, every subsequent attempt suffers a cumulative -2 penalty. This penalty is removed once the character takes some action other than attempting to immobilize that target.