Is the lead creation getting written to the Marketo log in Dynamics? Does the user creating the lead have the Marketo Sync User role?
Leads created in Marketo do not sync automatically to the CRM. You need to fire a "sync with Dynamics" flow step. You can do it manually on your lead or in a smart campaign. This is what enables you to control which leads flow to the CRM: you will be able to filter the smart campaigns that push leads to the CRM so that only your qualified leads qualify for the campaign.
Please note the opposite flow is automatic : any lead created in MSD will flow to Marketo.