Email cannot be received automatically

I have exch 5.5 sp3. Early this year, it works ok..when someone sends an email, the recipent will receive it automatically without having to click send/receive button. But these few months, the users will have to click send/receive button or else they will not receive the mail. I think my Exch has a problem. Restarting the server is just a temporary measures..a few weeks later the problem will starts again.In fact after restarting it, receving email will not be not as fast as it used to be but not as slow as the user complaints.
Is there a properties in Exch that I have to refresh or adjust so that the user can receive email automatically again?
appreciate the feedbacks..