There are any number of reasons that this can happen - chat has failed to complete correctly, you have a dirty line, etc. So check your syslog for indications.
Look in the PPP FAQ (which is really a series of questions and answers) for relevant information. This is a very comprehensive document and the answers ARE there! From my own (recent and sad) experience, if the answer to your problems is not there, the problem is NOT ppp's fault! In my case I was using an ELF kernel that I had not upgraded when I had upgraded the kernel modules. I only wasted about 2 days (and most of one night) cursing what had been a perfect PPP server before the light dawned!