If purchase membership button isn't there, the URL is bad or blank. Update the URL for the button. 

Program Forms (Scheduled Programs): 
  1. Click Web > Manage Program Forms
  2. Expand arrow for event 
  3. Click Options
  4. Mark the checkbox for the Include a button to direct ineligible users to another page field
  5. Enter membership page URL in Direct ineligible users to this page field
  6. Click Save

Event Registration Forms (Pre-Registered Programs): 
  1. Click Web > Manage Event Registration Forms
  2. Expand arrow for event 
  3. Click Options
  4. Click the Language tab
  5. Choose Membership - Not eligible to purchase information from the Category drop down field
  6. Enter membership page URL in the Direct ineligible users to another page URL field
  7. Click Save

In other scenarios, the webform layout may not be responsive.

Update your layout to be responsive
  1. Click Web > Page Designer
  2. Select the template matching yours followed by , ResponsiveUser-added image
  3. Click Next until you get to the final page, then click Submit