Disable mouse over event while holding left button,enable when release.
It seems that Left Mouse Button event never works.
Is there any other way?
This is not a complete answer, as it should be working as you expect (Maybe setting the status every tick is screwing something over?) but can be improved quite a bit, and at the end, it will be easier to spot why it is failing Let me explain:
When dealing with user input, it’s better to bind a new button to a custom action event. (Settings > Project Settings > Input > Action Mapping ) This is how mine looks on a test project:
http://i.imgur.com/0YKarOL.png
After that, you can use the event as you would with a normal player key press, but this time, they can edit it on the final version, which is always nice.
Now, for the implementation, your idea is not the best one, performance wise. Instead of checking every tick, just change it when you actually press / release the key, like this:
http://i.imgur.com/40XX1EU.png
With these ideas on mind, you should be able to improve the flow (and performance of the project) and it will be more likely for you to spot the errors!