I’m using the free version with a Gmail POP account that receives a lot of email. When I open emClient, it downloads a number of emails and then stops. If I hit Send and Receive, it downloads the same number again. Sometimes I have to hit Send and Receive five or six times to download all of the emails. The problem is not Gmail because I tested the account on Thunderbird, and it doesn’t exhibit the same behavior. Is this limitation with the free version only, or does the paid version have the same problem? 

this is definitely not a designed behaviour and changing to PRO will not resolve this.
Is there any error message in Tools>Operations window when you download your messages?
Can you tell me what version of eM Client are you running? (Help>About section)