loosing emails

When I go in to my emails to check for new posts, the first two or three new emails just roll off the top of the list never to be seen again. So when my list of new emails come in, the first two or three are gone and cannot find them in any other folder. Have checked deletes, junk, etc and they are gone. most of the time they aren't important, they might just be an ad or something but occasionally I have missed or lost an email sent that was important. anyone know why this happens?? thanks