Generally it's either or; Chances are you may have added songs that are larger that the average size of the songs they used to measure. For example, I've added songs that are 24 minutes long, and can clearly not be the same size as a normal length song.
If you are looking at 1 gig and 250songs, you are talking about average of 4 minutes per song. Then you need to consider other factors such as: Have you recorded anything on it? did you put any other files on it? How large are the files that the MP3 player needs to function?
If there is a way to see how much room you've use and how much is left (It should generally show you when you are trying to add more songs) then make sure you aren't simply full.
Even if it says it CAN hold 250songs, once the memory is full...it's just plain full.