If probation told you three to four months, o would go with that. Why you should do is call probation very few days and politely ask them about your status until something is resolved. If you don't get any satisfactory answers after a few weeks, hire an attorney or contact the public defenders office in your county. Good luck to you.
I generally with Mr. Pullman but this time perhaps he didn't notice that's it's already been 4 months. Calling them every 3-4 days will only make you someone they want to avoid. Hire a lawyer to check out the backlog for you.
I happen to practice in Fresno County and I have had similar problems contacting juvenile probation officers. The good news is that even without sealing your record, nobody can see the conviction with the exception of the District Attorney because juvenile convictions are not a matter of public record.
As for getting your case sealed, I would recommend that you go out to the Juvenile Justice Center and wait in the probation office waiting room until you get to speak with someone. Those people are masters at avoiding people on the phone, you may never get in touch with them. Good luck.