put it in the fridge until its cold, then try again. if not add a drop (but only a drop) of nail varnish remover to the bottle and shake around. that should do the trick.
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Yeah you can buy nail polish thinners that can make your polish runny again! if u cant find a polish thinner then you can put abit of polish remover into the bottle give it a shake n hey presto! GOOD LUCK!!