Hello Martyna,
There are many factors that prevent the tooltip from showing up. So it is hard to tell you what is not working and what could be done without taking look at your code.
Could you reproduce a
working demo in the online editor that I could work on? And if you use
styledMode you should include this in your demo.
You can start here:
https://jsfiddle.net/BlackLabel/e2x9hoam/
Best regards!