not recieving all of my emails

hi, our business email used to be on Outlook express but we had to upgrade our computer and thus have had to start using windows live mail. since then we know we are not getting all of the emails that are being sent to us. one example is we watched someone send an emai to us then we went back to our business office and the email wasnt there. we are still recieving some email but not all. i have checked a few things such as junk mail folders (not in there) made sure the 'show all emails' was selected in view and made sure conversations was turned off so they didn't get grouped. we are losing business because of this and need a solution.

thanks for your time