I asked a friend at PBS about this theory that the first  season shows don't air as often as later seasons and it doesn't seem to be true. The latest season released gets extra airtime and then once a newer season comes out that previous season gets put into the same rotation as the other shows.
So season one shows will be aired as many times as season 15 on average. On average just becuase of where they are in the rotation at the end of the year that you are used for counting.
So the kind of numbers she gave me from looking at the broadcast shedule from Mid Sept 2012 to Mid May 2013 are shows from season 16 will be aired 68 times and shows from season 4 will be aired 20 times. But season one and season 4 airing numbers are the same.
There is some distortion due to the shows that are aired for special events such as the special airing of April 9th in the aftermath of the Conneticut massacre 2 month ago. Also some things like the elections or inauguration or olympics might have some special shows brought into the line up for those weeks.

So  it's just a matter of patience if you haven't seen season one for a long time.
Sometimes broadcasters publish their future broadcast dates well in advance if you look for it on the website. I remember seeing BBC one once and it listed the dates of upcoming Arthur shows well in advance.
I haven't checked for the PBS broadcast schedule.
On a side note the way the shows are paired does not change.