As it's been the case ever since we introduced this feature over ten years ago, the button is a link with the URL to start the optimisation process. In version 6.1.2 it is something like https://www.example.com/administrator/index.php?option=com_admintools&view=DatabaseTools&task=optimize&tmpl=component
. There are attributes on the element which are caught by JavaScript on the page to actually open this page in a modal dialog, inside an iFrame.
If you no longer see the modal it means one of two things. Either you need to clear your browser's cache or a third party plugin is causing a JavaScript error which prevents our own JavaScript from running as well (browsers will stop executing JavaScript after the first error they encounter when parsing a page).
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!