Welcome, elainmir.
I am assuming you are using the stock Android email client:
There are known issues with this client, and the one you report is one of them. I have the same issue.
You can restart the phone, and this will cause the email to sync again temporarily; however, usually after a day or two it stops syncing again.
You can try using the K-9 email client from the market. Some people say it works better, others say it doesn't solve the issue and/or is buggy in other ways.
I gave up on the email clients and now go to m.comcast.net via the browser. This has the advantage of allowing you to delete unwanted emails from the server. However, it isn't automatic, you have to remember to go there.
Wish I had a better solution for us, maybe someone else will.
Edited to add: many people set up their other email accounts to forward to their gmail account so as to use the gmail client, which works much better. I chose not to do this, but you may consider it.