Ipod Question: What is going on with the ITunes to Ipod transfer?

I am getting a new Ipod…that’s settled. However, I noticed this with another Ipod I had before as well and am hoping some tech-savvy person can let me know what is going on.

I have an 8 GB Ipod Classic that I have been using. I mostly download radio shows and audio books. I put the downloads onto my hard drive then export them to ITunes and then onto my Ipod. I was able to probably download about 50 to 100 radio shows and audiobooks. As time wears on, I have to “uncheck” more and more shows/books to put ONE show on. That is, I have to eliminate exporting off ITunes onto my Ipod more shows (say four or five three-hour shows to simply get ONE show on.) So, now, my Ipod is only holding a handful of shows (about a dozen) when it used to hold 50 shows! The shows are still on ITunes, okay? I just cannot put them onto my Ipod….there isn’t enough space on my Ipod (it says). So, I have most shows “unchecked” on my ITunes and cannot get them on my Ipod anymore. My Ipod now only holds half a dozen shows! And won’t have take anymore.

I am absolutely confused. Am I missing something? I hope someone can help me understand this.