I have seen this post over the years as this has happened to me a few times in different versions of SQL Server. I have found the following has resolved the symptoms. Open the Job Schedule Properties. See the Duration section and reset the Start date: to today. Click Ok to implement the edit. Then Right Click the SQL Server Agent node in the Object Explorer and choose Restart. Repeat these steps for each schedule that is not getting kicked of automatically.

I am not discounting any of the other suggestions such as ms patches etc. and I don't think we will ever know the root cause. I am guessing that there is a catalog that fails to get updated properly or a cache of sorts that gets corrupt when making edits to an existing job and or schedule that throws the ss agent off. Just wanted a good place to document one workaround.