Computer Hope

I have had this issue a few times. I have a gmail account and I would like to b able to check mail from another gmail address on it. I successfully managed to do so with a couple of accounts. However, I am still struggling to link others to it. I keep getting the following error message:

Server denied POP3 access for the given username and password

I have had this issue with a couple of gmail accounts. Please advise how to fix this problem!