iPod Shuffle not playing all songs loaded.

I purchased two refurbished 512k 1st generation iPod shuffles from an online retailer. When I received them one of them appeared to be a 1G shuffle instead of the 512K that I ordered. This iPod shows a capacity of 992.8MB in Itunes while the other shows a capacity of 489.8MB. I loaded aproximately 35 songs on each iPod and it appeared that both loaded fine. However, when the songs are played on the larger capacity iPod only about half of the songs actually play. I have tried restoring the iPod in Itunes and I still get the same effect. All songs are showing as properly loaded in Itunes. Is it possible that the internal workings of the shuffle are built at 512k and therefore not able to access the second half of songs? The iPod is running software version 1.1.5 and I am running Itunes version 7.5.0.20