This might sound basic but a sometimes it’s overlooked. Please make sure that your theme actually has sidebar widget area in the post, some themes doesn’t have sidebar by design (the so called one-column theme), but allow the widget area in the footer. Also check theme options, see to it that theme has the option to display post/page with or without sidebar. Some themes have Post template and Page template for users to select, and this usually included the no-sidebar or with sidebar template.
If all the above isn’t the case, then there might be theme/plugin conflicts. Try disabling all plugins and see if problem goes away, if it works then re-enable them one by one to rule out the culprit. If no plugins seem to be the problem, try changing theme, switch to any of the WP default themes and see if problem persists.
