Its a bit hard to answer such a vague question - so if you can ellaborate a bit on the customization you have done in your JIRA instance, it can help.
Some pointers:
JIRA is using workflows to manage the flow an issue can use.
The default workflow that comes with an installation of JIRA is called "jira" and looks like this: http://www.atlassian.com/software/jira/docs/v3.13/default_workflow.html
Depending on the customization of your JIRA installation, you can either change the workflow to go from Open to Closed when you use the "Resolve" operation.
If you are not the JIRA administrator, different workflow transitions can have various properties and conditions and validators set on them, for example checking for permissions. So if you are a user that does not have permissions to do a certain transition - you can see if you can change it via the permissions administation panel, or again - in the workflow validations.