As I presume you have checked the bugzilla and you give too little information no one can answer why.
It happened this way:
suddenly emails came in with one day delay. I found in the header, that the emails had rested for a whole day at SME mailserver. I tried to change all settings and changed all back. After doing this fetchmail retrieved all emails from the ISP.
Then emails were not retrieved for some more days.
Finally I uninstalled an reinstalled all fetchmail. Now fetchmail fetches none of the emails any more.
I'm thinking of reinstalling all SME as a solution.