Help Mobile Internet not working


Hi, can anyone help me with a problem I am having with connecting to the internet via the mobile data network? Basically I have full signal and the up light at the top of the screen lights up, but not the down arrow. I have no problems with the wi fi connection and I can connect from time to time on the moibile date network, but it just doesn't work all the time. Anyone else have this problem, and if so have you managed to fix it?