Responsive design issue

Im having an issue with the responsive design when the width of the site is between 420px and 767px.

The search bar located in my left side bar, is merging with the main center content. Either the side bar should shirk/ adjust to the small width or disappear and the view should change to the mobile view between these values.

I have also noticed that any sidebar widgets placed underneath the advanced search box are contained within the search box and not in their own container.

Please can you provide a fix for this issue.

TIA