After years of having no problems I am unable to dowload recently purchased music onto my iPod Nano

After years of having no problems I am unable to dowload recently purchased music onto my iPod nano, each piece of music purchased has a tick on the left hand side. We bought a new computer last summer and downloaded ITunes onto it and the iPod was authorised. We have the up to date version of ITunes which no longer appears to have a 'button' at the bottom to click on to download the purchases onto the iPod.