With Google Tag Manager issues we usually find that the problem is actually arising from one of the scripts (or tags) that GTM is being used to include.
In this case, we identified with Alex that it’s Olark chat widget. We’ve seen customers experience this before Ð for some reason, Olark interferes with the enter key handler on all forms on the page. It ends up reloading the page before Unbounce has had a chance to store the lead data and perform the form confirmation action.
Sadly we don’t have a fix at the moment other than to remove the Olark script from your pages that use forms. We’re going to look into this though Ð and I’ll reply here when we have an update.
As an aside, if anyone is interested in the implications of using GTM events (gtm.click, gtm.formSubmit, etc) with Unbounce, check out my reply here.