Months notice on cancellation not honoured

In June I cancelled my portable possessions insurance as my premium had increased. I was told that the changes to my policy would only be effective in August as payments are deducted a month in advance and are not adjusted on a pro-rata basis. The payment for July had already been made. In July my laptop was ****** from my car. I filed a claim which was then rejected because my laptop had already been removed from my policy.