shortcode works OK on Pages, not at all on Posts

I’m an experienced PHP coder and have created a custom theme from scratch and everything is working OK however my shortcode only works on Pages but not on Posts.

Does anyone have a hint of what I should look at?

Here’s an example in my functions.php file:

function show_pmpa_registration($atts, $content = null) {
extract(shortcode_atts(array(
"amount" => '89'
), $atts));
global $post;
$retour = "Hello world";
return $retour;
}
add_shortcode("register_pmpa", "show_pmpa_registration");


When I add the following shortcode in my Post it does not work and just echoes the shortcode:


[register_pmpa amount=99.99]


Thank you.