Contact Form 7 Spinner missing

I’m running Contact Forms 7 4.6.1.

Recently the “spinner” generated by the Ajax Loader has stopped working. The form is correctly submitted when the SEND button is clicked, but the spinner does not appear. Has anybody else experienced this problem? It could be a plugin conflict, but I hate to go through the process of plug-in elimination if the problem might be somewhere else.

I’m running WP 4.7.2 with the Souffle theme (with child theme).

Thanks