nokia 5230 music issue!?

[Hannah]

i have recently brought a brand new Nokia 5230 , i also brought a Sandisk 4gb memory card and i checked it was compatable in the handbook , i put the memory card in the usb thing which goes into the computer to read the things on the card and add music to it etc. I added music i purchased from itunes and put it on the memory card and put the card back into my phone , the music came up and worked perfectly , i did it 5 times after and it worked great , but the next time i did it , it didnt work , i did the exact same thing , put it in the right folder and everything but when i put the card back in the phone none of the songs were on there , so i turned my phone of and on and removed the card and put it back in , then some of the other songs disappeared and others had error notices on them , i put the memory card in my sisters phone and she has the same phone and it worked fine , all the songs were there , i also reseted the factory settings on my phone and even removed all the songs of the memory card and put them back on but still they disappear and others have error notices. Im completely stuck , i purchased it from argos and can only take it back if it is faulty same as the memory card , does anyone know how to help , before i take the phone back ? any ideas will be appreciated ! =]