I have created a purchase form which I've hooked into every post on a site (3500+ posts). A checkbox expands the form. On categort index/archive pages only the first post shows the checkbox. To see the form on all other posts you have to actually call up the post itself.
The site displays all posts in full. The form displays within posts perfectly and functions as expected. I've tried adding the form as a shortcode or function call - no difference.
If I add plain text into the hooked area then it displays fine on all posts - only the form has the problem.
Is there some reason that only the first post shows the checkbox and not subsequent posts?