Doesn't sticky for some browser widths

Hello,

First of all I love the plug-in and works perfectly, except I have one little issue.
On the option, I have set “screen max width” to 650px, which is when mobile view begins.
But I noticed that from 651px to 667px, the widget still does not stick. From 668px, it works beautifully.

How can I resolve this issue? If you need to see it for yourself, my site is http://sdherald.com.

Thank you,
Andrew

https://ww.wp.xz.cn/plugins/q2w3-fixed-widget/