I use infopath to design the forms and used a people picker.SharePoint Designer workflow sent mails to all the users selected in it picker. The mail only goes to the first recipient. How do i sent emails to multiple users using sharepoint designer workflow.
Check what type are you using in Designer workflow for People picker field? Is it string? Try selecting email or username delimited with semicolon(I exactly don't remember the attribute).
Even then if its not sending to all, log the people picker field value. Check if you are able to log all values or only the first value. This will give an idea as something is wrong in declaring the value.