...Which means that you probably have it disabled within an event somewhere, like I mentioned above.

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If you have RM2K3 (like you should) - Go view the event which calls the battle. Highlight the Enemy Encounter line and hit space. Change the Escape Handler to End Event Processing.

If you have RM2k, you might have disabled escaping somewhere eariler and forgotten to re-enable it. I forget the exact command to enable/disable escaping from battle, but you'll want to make sure to enable escaping. If you did a boss battle eariler on, check to make sure you re-enabled escaping from battle afterwards.

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The only other way I know of for it to be disabled is if you're in a Pincer Attack (situation with 2 or more enemies surrounding you). Escaping is disabled for the whole battle.