Why do I keep receiving emails about my account being at capacity?

I keep receiving emails about my account being at capacity. It is no longer at capacity. One form was deleted, so more room should have been made.

Deleting forms does not reduce the number of submissions you have received for the month.  It will reduce the number of submissions you have stored in your account, though the number you have received for the month will remain the same. 

There are several "Get More" tasks that can be done in your account settings to permanently increase your limits:

https://www.jotform.com/myaccount/getMore

The limits will also reset on the first day of each month.

Is there a way to undelete forms?
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If the form was not fully purged from your trash, yes, the form can be restored:

http://www.jotform.com/help/134-How-to-restore-deleted-forms

However, if you purged it from the trash, it cannot be restored.