button shortcode – text inside not responsive

Using your button shortcode. Each button contains text. When the page is viewed full screen, the text stays within the button properly. When viewing the page with reduced width OR on smartphone, the text within the button does not remain within the button – some spills out of the button and below it. Please see two attachments. How can I make the text always remain within the button (fully responsive) regardless of width of the page or viewing device? Thank you. Stuart