fixed not fixed

Hello! i create a mobile button – to direct call this button in right bottom corner in the mobile screen size (and hidden if more then 480px wide) (create style – as usual – but dont working!) i know – you there not for solve problems like this. But my question – how you can write css rules – that in mobile phone – “position:fixed;” – dont work! i dont saw this problem early anywhere.
What starange: this rure (position:fixed;) works fine on desktop wide screen – but only when mobile – fixed – not fixed. absolute instead.Sorry my english i hope you understand me.

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