Unusable within 6 months

I bought this with high expectations as I usually love Essie nail varnish. The product performed reasonably well, but 6 months after opening I found that the consistency changed drastically to overly thick, and appears to be full of air bubbles when I try to use it. I don't paint my nails often at all, and most nail varnish bottles last years as I store them in a cool dark place. Not sure what went wrong with this one.