Why wont my ipod touch 4.2.1 download apps?

i try to download apps but it wont let me? i have had it for about a year now and it hasnt ever had any other troubles?but anyways this is whta happens..ill go to my app store and press install and i put in my password then it will pop up to my ipod screen saying waiting... under neath the app and then the app will just go away and it wont install?? can someone please help me!?!!