Missing emails

I am not receiving all of my emails in my comcast.net account. I get some. They are usually late. For instance, event at 6pm cancelled. I receive the email the next day. Other emails from this same sender never make it to my inbox. I am using Mail for iphone and the xfinity website. I also check my spam multiple times during the day. The emails are just not there. How can I get comcast to release these missing emails? I am receiving other senders' emails with no issues.