Hi @Atteeq, welcome to the Community! 
Sorry to hear of the troubles getting the desired Marketo form appearing in the trigger for your Zap. I can see that you also reached out to the Support team about this and they identified that you’re running into a bug with the New Form Submission (Marketo) trigger. There’s no updates on the bug at present and I can’t provide any sort of estimate as to when it will be resolved by, but you’ll get an email notification from us as soon as it’s fixed.
It’s been noted on the bug report that manually selecting the form by using the Custom value option and entering it’s ID doesn’t work. So as an alternative temporary workaround, can you get an email notifications sent to your email account from Marketo when the form is submitted?
If yes and those emails contain the desired information from the submission, can you try using Email Parser for the Zap’s trigger instead. You’d need to set up automatic forwarding in your email account to automatically forward the form submission emails to an Email Parser mailbox. Then in the Zap you’d use the New Email (Email Parser by Zapier) trigger so that it triggers when a new form submission email is received. And once the bug is fixed you can go back to using Marketo for the trigger. 
If that sounds like a viable workaround you can learn more about how to set up Email Parser here: Trigger Zaps from new parsed emails
Hope that helps. Please do keep us updated on how you get on with this, keen to ensure you’re all set!
